My local camera shop had a preproduction A7R III and a 24-105 in for folks to see, and I was fortunate enough to have had the opportunity to play with it for about half an hour. Here are my observations and first thoughts, keeping in mind that it was a preproduction unit with early firmware. They did not permit me to put a memory card in the camera to keep any shots 
1. Viewfinder - The viewfinder was easily an order of magnitude better than A7R II. Mostly gone is the jello effect, and it is actually quite pleasant to use for subjects that are still, or for subjects that are slow-moving (like walking). It's very sharp and very clear.
The viewfinder still blurs when you pan the camera. For example, I pointed it at a large sign that said SONY, and zoomed in such that the letters filled about 3/4 of the viewfinder horizontally. When I panned even slowly, the letters suffered from visible motion blur. Not as badly as A7R II, not even remotely close to the experience of an optical viewfinder.
In addition, for some reason, the viewfinder started to stutter in jarring (unusable) way when the camera was in continuous autofocus. When I pointed it out to the Sony guy, he reset the camera, and that went away. He blamed it on preproduction firmware, which is entirely plausible. He also said that they had a number of other issues with that unit.
2. Autofocus - On the 24-105, autofocus was smooth, quick, and apparently accurate when using people as subjects. I say apparently, since there's no way to tell with no memory card. But, it didn't stumble, and the experience was favorable. The default intelligent mode, where the camera seems to figure out what you might want to focus on is kind of crazy, because it seemed to just randomly grab a person walking by, when I was clearly pointing at a tripod. And if there's multiple people in the scene, I have no idea how it figures out who to focus on. Where there was only 1 human, it always reliably identified the person and set AF to the face (AF face detection was on).
The Joystick is a very, very welcome. It worked well when the camera was set to center point. When nonhuman subjects were far away (15 ft+), AF seemed to be quick and responsive with centerpoint. However, when objects were close (3-6ft), center point sometimes did not focus on what I wanted it to. It was quite reproducible, and the explanation was that this was a preproduction unit.
I was also permitted to attach a G-Master 70-200/2.8. On the plus side, the lens on the camera feels very balanced, and the rig feels nice. But the good news ended there. The autofocus was horrible - it hunted a LOT, at times unusably so for a subject like a price tag at about 6 feet. It would actually go back and forth and stutter like it was having a seizure, never locking on. Again, this was blamed on preproduction.
It did seem to AF better at f/2.8 than stopped down to f/5.6. Could it be that there wasn't enough light? Either way, it would not be acceptable to me in a production unit.
3. FPS, Buffer, Uncompressed vs Compressed RAW - The Sony guy tried to sell me on Compressed RAW, which lowers it to 12 bit (from 14), and also drops the file sizes from 100MB / file to about half that. He claims that the differences are not detectable, but of course, there's no way to without recording photos. He told me 26 frame buffer in uncompressed mode, and seventy-something (76?) in compressed, either of which is very impressive, especially considering the FPS. 100MB / file... aie carumba. 600 photos, and a 64GB card would be full LOL.
He mentioned that he filled a 128GB card pretty quick. So, it's worth thinking about before buying a 40+ megapixel camera, the cost of larger memory cards, since 128 will be the new 64, and if you want them UHS2, that adds to the total cost of ownership.
4. Buttons, Build, and Egonomics - There are a lot more buttons, which is very welcome, and a rear dial, of course. However, on the down side, there are still way too few buttons. There are many, many things that Canon cameras have dedicated buttons for, like changing AE, Focus, ISO, Drive, etc. -- and on the Sony, the answer is "program it to one of the custom buttons". But the problem is, there aren't nearly enough, at least for me. The dials also had a certain, non-industrial cheapness to them, in comparison to Canon or Nikon dials, but maybe that's just me.
In terms of ergonomics, it's feels just like the A7RII. So if you like that, you'll like this, and if you didn't you'll still hate it. There are a million threads about Sony A7RII ergonomics, so I'll just skip right past it, except to say that personally, they have always felt ok with smaller lenses, and awkward with anything larger than a 70-200.
I guess it's to be expected since the build hasn't changed, but the A7R III still feels like a delicate flower. I think I would hesitate to use it in a situation where I might drop it, because it doesn't feel like it would survive that kind of handling. Or, bashing it against something accidentally. Though uncommon, it's not like I've never done those things to my cameras.
5. Techno-gizmo-features - This camera is PACKED with them. There are so many features that I didn't even know where to start to explore them, and I certainly wouldn't do any justice trying to pass on my observations, because I really didn't know what I was doing. How many of those features would actually be useful, I have no idea.
Oh, it had 2 card slots, Yay. On the minus side, the remote trigger uses a mini USB -- it shares this with the computer connection. I say that's a minus, because this is much , much more fragile than mini mic/N3, and it certainly is not water resistant like N3. On the other hand, it doesn't seem like the A7R II survives water very well, so maybe this is more a fairweather camera. Plus, you can't tether AND use a remote trigger, right?
6. Other things mentioned - The battery seems much improved, and they brought up 5.5 stops of IS, over 4 stops, which is pretty amazing. He also brought up more dynamic range, claiming 15 stops. Really? I thought the sensor output was not much different than A7R II. Whatever. I just thought it was cute that this was on the list of things to rattle off to potential customers. Along the same lines, he also brought up some HDR stuff, which is also unexciting to me.
Ironically, everyone there universally suggested that I buy the Sigma MC11 adapter to use with the Sony body so that I could preserve my Canon lenses. The camera shop guy pointed out that comparable Canon lenses were MUCH cheaper, and the Sony guy chimed in that with the Sigma adapter, all of the advanced Sony features would work, and that he preferred this over the metabones adapter that was twice the price (wait, so he uses Canon lenses with his Sony?).
The Sony guy mentioned that on the video end, there were advantages to using the native Sony lenses, but since I identified right at the beginning that that I didn't give a flying fart about video, he said that for photographs, Canon lenses would work every bit as well (as Sonys). Which was just kind of stunning to hear from him, I guess.
The Camera store person chimed in that this would also allow me to use the lenses with my Canons. Really? I would have never thought of that!
OTOH, I suppose Sony guy might have been a Sony champion of some sort, rather than a Sony employee. He was wearing a Sony shirt, though 8)
Final thoughts -- well, at this moment, even if I really wanted one, after seeing the preproduction unit, I sure wouldn't buy one, until it came out and I was sure the bugs were all ironed out. Boy, were there a lot, for a unit that they were demonstrating to the general public. Maybe I'm being unfair, but it feels like it's rushed to make Christmas.
That said, would I buy one, if it were bug free? There are a lot of things to really like about it, but I think the EVF still has to get a little better before it would be useful -- or at least, not migraine inducing -- for my style of birding. I would need to see AF on the final version, and it would have to be a lot better on lenses that I'd use before I'd consider it, too. At the price, I don't think that these things should have to be compromised on.
The remote shutter might kill it for me too, because I use a wired remote trigger a lot (no batteries), and micro USB is not a good connector for that, in my opinion. Plus, I do not like the idea of not being able to tether and have a wired trigger at the same time.
Would I buy one if it were a Canon, in a Canon body, with Canon ergonomics? Maybe. Which is saying a lot, because (a) it's more than I would think of spending on a body and (b) because the EVF still isn't as pleasant to use as an OVF, and having fun shooting is very important to me in photography as a hobby. Still, it gives hope for me as a mirrorless customer, I guess!
Still, it was lots of fun to check out and well worth the half hour.
1. Viewfinder - The viewfinder was easily an order of magnitude better than A7R II. Mostly gone is the jello effect, and it is actually quite pleasant to use for subjects that are still, or for subjects that are slow-moving (like walking). It's very sharp and very clear.
The viewfinder still blurs when you pan the camera. For example, I pointed it at a large sign that said SONY, and zoomed in such that the letters filled about 3/4 of the viewfinder horizontally. When I panned even slowly, the letters suffered from visible motion blur. Not as badly as A7R II, not even remotely close to the experience of an optical viewfinder.
In addition, for some reason, the viewfinder started to stutter in jarring (unusable) way when the camera was in continuous autofocus. When I pointed it out to the Sony guy, he reset the camera, and that went away. He blamed it on preproduction firmware, which is entirely plausible. He also said that they had a number of other issues with that unit.
2. Autofocus - On the 24-105, autofocus was smooth, quick, and apparently accurate when using people as subjects. I say apparently, since there's no way to tell with no memory card. But, it didn't stumble, and the experience was favorable. The default intelligent mode, where the camera seems to figure out what you might want to focus on is kind of crazy, because it seemed to just randomly grab a person walking by, when I was clearly pointing at a tripod. And if there's multiple people in the scene, I have no idea how it figures out who to focus on. Where there was only 1 human, it always reliably identified the person and set AF to the face (AF face detection was on).
The Joystick is a very, very welcome. It worked well when the camera was set to center point. When nonhuman subjects were far away (15 ft+), AF seemed to be quick and responsive with centerpoint. However, when objects were close (3-6ft), center point sometimes did not focus on what I wanted it to. It was quite reproducible, and the explanation was that this was a preproduction unit.
I was also permitted to attach a G-Master 70-200/2.8. On the plus side, the lens on the camera feels very balanced, and the rig feels nice. But the good news ended there. The autofocus was horrible - it hunted a LOT, at times unusably so for a subject like a price tag at about 6 feet. It would actually go back and forth and stutter like it was having a seizure, never locking on. Again, this was blamed on preproduction.
It did seem to AF better at f/2.8 than stopped down to f/5.6. Could it be that there wasn't enough light? Either way, it would not be acceptable to me in a production unit.
3. FPS, Buffer, Uncompressed vs Compressed RAW - The Sony guy tried to sell me on Compressed RAW, which lowers it to 12 bit (from 14), and also drops the file sizes from 100MB / file to about half that. He claims that the differences are not detectable, but of course, there's no way to without recording photos. He told me 26 frame buffer in uncompressed mode, and seventy-something (76?) in compressed, either of which is very impressive, especially considering the FPS. 100MB / file... aie carumba. 600 photos, and a 64GB card would be full LOL.
He mentioned that he filled a 128GB card pretty quick. So, it's worth thinking about before buying a 40+ megapixel camera, the cost of larger memory cards, since 128 will be the new 64, and if you want them UHS2, that adds to the total cost of ownership.
4. Buttons, Build, and Egonomics - There are a lot more buttons, which is very welcome, and a rear dial, of course. However, on the down side, there are still way too few buttons. There are many, many things that Canon cameras have dedicated buttons for, like changing AE, Focus, ISO, Drive, etc. -- and on the Sony, the answer is "program it to one of the custom buttons". But the problem is, there aren't nearly enough, at least for me. The dials also had a certain, non-industrial cheapness to them, in comparison to Canon or Nikon dials, but maybe that's just me.
In terms of ergonomics, it's feels just like the A7RII. So if you like that, you'll like this, and if you didn't you'll still hate it. There are a million threads about Sony A7RII ergonomics, so I'll just skip right past it, except to say that personally, they have always felt ok with smaller lenses, and awkward with anything larger than a 70-200.
I guess it's to be expected since the build hasn't changed, but the A7R III still feels like a delicate flower. I think I would hesitate to use it in a situation where I might drop it, because it doesn't feel like it would survive that kind of handling. Or, bashing it against something accidentally. Though uncommon, it's not like I've never done those things to my cameras.
5. Techno-gizmo-features - This camera is PACKED with them. There are so many features that I didn't even know where to start to explore them, and I certainly wouldn't do any justice trying to pass on my observations, because I really didn't know what I was doing. How many of those features would actually be useful, I have no idea.
Oh, it had 2 card slots, Yay. On the minus side, the remote trigger uses a mini USB -- it shares this with the computer connection. I say that's a minus, because this is much , much more fragile than mini mic/N3, and it certainly is not water resistant like N3. On the other hand, it doesn't seem like the A7R II survives water very well, so maybe this is more a fairweather camera. Plus, you can't tether AND use a remote trigger, right?
6. Other things mentioned - The battery seems much improved, and they brought up 5.5 stops of IS, over 4 stops, which is pretty amazing. He also brought up more dynamic range, claiming 15 stops. Really? I thought the sensor output was not much different than A7R II. Whatever. I just thought it was cute that this was on the list of things to rattle off to potential customers. Along the same lines, he also brought up some HDR stuff, which is also unexciting to me.
Ironically, everyone there universally suggested that I buy the Sigma MC11 adapter to use with the Sony body so that I could preserve my Canon lenses. The camera shop guy pointed out that comparable Canon lenses were MUCH cheaper, and the Sony guy chimed in that with the Sigma adapter, all of the advanced Sony features would work, and that he preferred this over the metabones adapter that was twice the price (wait, so he uses Canon lenses with his Sony?).
The Sony guy mentioned that on the video end, there were advantages to using the native Sony lenses, but since I identified right at the beginning that that I didn't give a flying fart about video, he said that for photographs, Canon lenses would work every bit as well (as Sonys). Which was just kind of stunning to hear from him, I guess.
The Camera store person chimed in that this would also allow me to use the lenses with my Canons. Really? I would have never thought of that!
OTOH, I suppose Sony guy might have been a Sony champion of some sort, rather than a Sony employee. He was wearing a Sony shirt, though 8)
Final thoughts -- well, at this moment, even if I really wanted one, after seeing the preproduction unit, I sure wouldn't buy one, until it came out and I was sure the bugs were all ironed out. Boy, were there a lot, for a unit that they were demonstrating to the general public. Maybe I'm being unfair, but it feels like it's rushed to make Christmas.
That said, would I buy one, if it were bug free? There are a lot of things to really like about it, but I think the EVF still has to get a little better before it would be useful -- or at least, not migraine inducing -- for my style of birding. I would need to see AF on the final version, and it would have to be a lot better on lenses that I'd use before I'd consider it, too. At the price, I don't think that these things should have to be compromised on.
The remote shutter might kill it for me too, because I use a wired remote trigger a lot (no batteries), and micro USB is not a good connector for that, in my opinion. Plus, I do not like the idea of not being able to tether and have a wired trigger at the same time.
Would I buy one if it were a Canon, in a Canon body, with Canon ergonomics? Maybe. Which is saying a lot, because (a) it's more than I would think of spending on a body and (b) because the EVF still isn't as pleasant to use as an OVF, and having fun shooting is very important to me in photography as a hobby. Still, it gives hope for me as a mirrorless customer, I guess!
Still, it was lots of fun to check out and well worth the half hour.
