Ive been a pro for over 15 years.
I would just like to say that I love my canon gear. It has never once failed me shooting daily and it takes the abuse and keeps shooting. Thats what I need and thats why I shoot Canon.
I also love the glass and they have a lens for almost every eventuality.
I also shoot a lot of differing genres of photography. Motorsport, commercial, events etc so I have a range of cameras and each one has its uses. I currently shoot with 6DMKII 7DMKII 70D and have just received a 5DMKIV.
I actually went an interesting direction. I had the 5DMKIII but it was stolen and went to the 6DMKII. The 5 was in my mind almost the perfect camera with a few downfalls in IQ but I rarely had too much issue. The 6DMKII although has lost a card slot the AF system isnt as good the IQ in my mind is better in the usable range.
The cost to replace the 5DMKIII was around 2k at the time and the 6DMKII has all the features I needed from the 5 plus the tilt screen. A feature I have been craving from a FF body for years. I bought the 5DMKIII at launch so had it for 4 years, didnt really want to spend 2k for another when it feels like an old camera. The 5DMKIV although again improves the III it didnt do enough to warrant almost double the cost over its predecessor (at the time).
The 6DMKIIs IQ shortfalls only rear its head when you push it to the very limit and its high ISO capabilities are tremendous. As a pro I haven't got close to the limit yet. Im very much a get it right not fix in post kind of guy and if you shoot this thing right it does very little wrong. I think if you learned the trade and can master light in a given situation DR isnt as much of an issue as the internet makes out. If im in a situation where the light isnt good I wont just shoot it and hope for the best later like these stupid pulls internet reviewers show. Thats not why you employee a professional and any 'pro' that does this sort of work is pulling the wool.
I use a lot of light modification which generally means pulling 3-4 stops of shadows or exposure is void to me.
It has incredible colour and the bump resolution really helps all my lenses feel that touch sharper on this body.
The reason I went in this direction is that there is no doubt that the 5DMKIV is one of the best cameras around but it was twice the price at the time and I really didnt feel it was worth the money over the 5DMKIII and I was looking for something that would aid me in image making, allowing me to get images that were difficult with the 5DMKIII rather than just buying a newer body with slightly newer tech. The screen on the 6DMKII is that feature for me.
At around £3k for the 5DMKIV and I paid £1489 for the 6DMKII on release day. The slightly better shadow recovery wasn't really worth twice the cost to me.
Another reason is that it was the end of the wedding season and in the winter months I generally shoot lots of standard commercial imagery. I have taken a contract to photography PPE, all studio work and I have also been shooting pubs, bars and restaurants. This doesn't require shooting in adverse conditions mostly on a tripod and is super chilled in comparison to event photography.
The 6DMKII is far better because of the tilt screen. British pubs are tight and even with 16-35mm i find myself up against walls most of the time so the screen makes my life so much easier, same with getting slightly odd angles, no more lying on the floor etc. It really has made my life a lot easier, my back and knees certainly!
I also shoot motorsport and i took the 6D along and they have certainly tweaked the AF because it hits and hits really well especially when you set the AF up properly. Basically has the same options as the 5 series but no presets so if you know what you are doing you just dial it in. I had no issues at all with it and shot with my 7DMKII and came home with more in tack images with the 6 than the 7.
Granted the one card slot isnt ideal. In actual practice I have never had a single card go on me, ever! Shooting roughly 500k images on each body ive owned before retiring it. In my mind its not really a huge issue, but I look after my cards and swap them out for new ones fairly regularly. On the other hand shooting events it is not wise at all, as there is no way to go back. Like I said one camera is not the answer and I use different gear for different situations.
Now wedding season is coming back and my first one is in March I have bought a 5DMKIV because for weddings and events having dual card slots is imperative you cant mess around with a day most hope to be the only one.
After using the 5DMKIV I still dont think its an incredible upgrade over the III obviously it is an incredibly rounded camera the IQ is excellent and has lots of the newer features which makes day to day image making a joy. It is like others have said so easy to pick up and shoot and processing the images I can use the same presets which makes life so easy. The other thing that is nice is the body design with the AF selection etc although you can get around this with the top and circualr mode dial on the 6DMKII in tandom for AF selection the AF stick on the 5 series is far better and a lot easier to use in time sensitive situations.
The main thing is when you remove the upgrade in IQ and some of the newer features like GPS and WIFI it doesn't help me make hugely different images over the III. I dont look to the camera to produce miracles but there is no denying that certain features do make image making easier and when you shoot all day every day those features are very much welcome, save time get in and out quicker and make more money.
For this reason the 6DMKII is probably one of my favorite Canon cameras thus far. It is super underrated if you liked the 5DMKIII you would love the 6.
Just a shame the reviewers only focus on shadow pulls rather than the camera as a package for image making.
If you want to read some of my real world thoughts I did write make a thread with the initial impressions with examples etc shooting real jobs etc.
http://www.canonrumors.com/forum/index.php?topic=33212.0