Search my comments on the 6D. I feel I have covered it pretty well, as has of course Dustin (many of my comments are in his thread, but there are quite a few other threads). I've compared and edited the 6D's files with the 5D3's and the 1DX's...and have over 8000 cycles on my 6D.
Basically there are people on here who seem to feel the need to bash the 6D because it's not the 5D3, hence the 5D3 is what they bought. So to them, if you use a 6D, you aren't as serious a photographer as they are, no matter what type of photography you do. They aren't hard to spot.
It's not so much that the 6D needs "more" autofocus points...it's that they should all be "cross type". Honestly an AF sensor similar or identical to the 7D's would have been nice. But that would have put the price, and especially the performance...too close to the 5D3...and thus would create a competing product line...which was never going to happen.
Barring the above, I feel Canon should at least update the firmware or whatever runs the autofocus sensor/system's processing...to make it "handoff" to the center point, when all points are active...rather than avoid it...since the center point is the only really good point. (This is the nature of the problem mine has when I attempt to make its servo tracking **** up.) But it's something I am happy to live with though, because I know how to make the system work perfectly fine (by leaving only center point active while in servo mode).
Then there's supposedly an issue with its color, or white balance, or color gamut...or whatever...to me this seems more like a trumped-up nitpicking session by those who are prejudiced against the 6D. The color is fantastic, period. The luminance noise at the pixel level is world class, especially above ISO 1000.
As for the weather sealing, there is some, but not much. The 6D is not technically "splash proof". I suppose it's also less impervious to dust...but if you're shooting in a very dusty environment...dust will find its way into everything even on the best sealed cameras. What makes me cringe the most about dust, is when it gets inside and in between the lens elements. There's no cleaning that without disassembly, usually by a Canon tech.
In my opinion, if you value "low-noise-at-medium-high-ISO" image quality and are price-conscious...the 6D is a no-brainer. But if you don't mind a slightly larger/heavier body...if you need loads of speed (comparatively) and all the bells and whistles, and are happy to throw down the 50% price premium...and don't mind the large luminance noise grain structure in the images above ISO 1000...and/or if you mostly need to shoot a lot of video, especially ML hacked RAW video...then the 5D3 is the no-brainer. For many, buying both cameras has also worked out well.
This is a fair assessment. When it came time for me to upgrade, I compared the features and the feel in my hand and went for the 6D. I was not, and am not, disappointed. It works more than well for what I do 95% of the time.
I just recently acquired a 5D3 as a gift from a family member. Yes, I am extremely fortunate. I still love my 6D. I rarely use shutter speeds > 1/4000th, though I have sometimes in the past. A neutral density filter fixes that. I don't shoot video. I occasionally shoot birds and airplanes, but only occasionally. I can do that with the 6D but probably will do it better with the 5D3 and have more keepers. I also don't shoot in a monsoon, though I doubt the 5D3 would hold up that well either. For that you'd need a 1D X and a weather sealed lens.
I'm hoping that Canon comes up with a firmware upgrade to fix the tracking issue, though leaving only the center point active works ok.
The controls are a mixed bag. The "Play" button on the 6D is easy to find with your shooting thumb. It's mixed in with three other buttons on the left side of the LCD on the 5D3. There are some other pros and cons of each.
I'm going to keep both and I suspect I will use both. For what I do
, I would not have bought a 5D3 though having it is nice. I had kept a 60D to as a backup/second body for the few weddings and events I do each year. That's now going to eBay.
For my forays into the nightlife, I'll carry my 6D.