I bought a 6D based on the recommendation of everyone here, and hence I think I owe everyone a review based on it, which I believe people upgrading from a Rebel model would appreciate. I myself upgraded from a 550D, so I guess I would have a similar idea of what people are looking for. I'm not aiming for a scientific journal, but rather more of experience relating, so please don't flame me for stuff you don't agree on.
1) Ergonomics and size:
This is highly rated by everyone, so I would give my point of view. I find it a lot bigger. MUCH BIGGER. Heavier too. My fingers used to wrap around the grip completely, but now I'm quite sure I don't reach the end of it. However, I guess it depends on individuals.
It's quite different from my 550D, it takes quite a bit of learning. For example, learning all over again how to take bracketed shots and long exposure shots is a pain. However, I like how aperture has a dedicated button. I would expect ISO to have one too. It requires you to press the middle button among five buttons (there is a little bump on it so you will be able to know without looking) without spinning the dial, so it's slightly troublesome. I will prefer something like Nikon's mechanism, with two dials.
3) Image Quality:
Now, this is the best part. Image quality is AWESOME. It's such a big upgrade from my 550D, I can't get back to using it anymore. The details captured are awesome, with a sharpness that I can never get from my 550D. I used the same lens for both camera, and the effect is just extraordinary, with just a change of camera. (Tamron 28-75mm F2.
It has a soft feeling, where my 550D would have been quite harsh with background but with very sharp lines, hence the image is distinct with a certain dreamy feeling. Bokeh effect is even stronger and more pleasant. It's much better for both portrait and landscape shots compared to 550D. Cropping a photo is ABSOLUTELY fine too. It's awesome how much detail you can extract from a shot!
4) Shooting speed:
Upgrading from a 550D, this is another big plus. It's able to take more than 20 RAW images continuously without buffering, and I don't think adding another frame in one second would matter much to me. However, I'm not heavily into sports shots, so that might explain it.
Now this is something I miss quite a bit. I would prefer to have even a built in flash at times, but I guess it pushed me to get an external flash. However, it means extra weight and space. Boohoo. =<
6) Wi-fi and GPS:
Not too big fan of this. I take photos for trips and own pleasure, so I don't really use such options frequently.
I decided to separate this from image quality, as I think it deserves a corner of its own! ISO was such a pain in the bottom when I was using the 550D: nothing acceptable beyond 1600! However, this totally changed my game. ISO6400 is now a common thing, with slight noise within it that is considerably unnoticeable. With a little photoshop, nothing's impossible! It makes me much more comfortable of using aperture values around F8-11 at anytime, anyplace. It also means I am now becoming a manipulator of light! Awesome!
I don't do much of fast-paced stuff, so I haven't really pushed it to its limits. However, I have always been using the center point as autofocus point, so that might explain it too. Anyway, I have developed the habit of locking onto the subject first using the middle point then move away, so auto-focus has not been much of an issue. However, at times when I accidentally changed the auto-focus points, it does seem to not work as well (Accidentally? You're bound to have that mistake when you upgrade from a Rebel!) However, the center point is an awesome focus point: it really focuses in the dark as advertised! Accurately!
I love being in the full-frame family. Image quality is awesome. Those who said that 6D is just a full-frame Rebel with no difference in image quality should try it. My friends described me being poisoned by it, and they too now have been poisoned by it! Trust me, constantly observing photos produced by my 550D and now looking at my 6D, I am a happy man. I do not know about D610's image quality nor have I tried it out, so it's a hard comparison for me, but as an upgrade, definitely not regretting it. It expands the arsenal of my shots and skills. I hereby declare that for me, equipment does make a difference.