I've read a lot of comments and about every review....here's my 2 cents.
I've had my 5d3 for one week now...I'm luke warm on it. 50/50 that it will be sent back and I''ll stick w/ Nikon.
I decided to switch from the Nikon d7000 because I wanted to upgrade to full frame and when I placed my order I was sure that the D800's 36mp would limit it's low light capabilities....it seems I (many others) were wrong if you look at some of the reviews on the d800 it appears that it has great low light ability, ON TOP OF JAW DROPPING DETAIL....the 4 fps is my only sticking point.
The other problem for me is when I compare the performance of my new 5d3, it's just not that much better image quality than my nikon d7000....a little better, but by only a very, very small amount...maybe 3% better if I had to put a number on it. - and only when pixel peeping. ALSO...I'M SORRY TO SAY THIS BUT THE NIKON D7000 HAS AN AUTOFOCUS SYSTEM THAT IS JUST AS FAST AND ACCURATE AS THE 5D3.....and much better dynamic range....I say this in frustration, I thought that a $3500 "latest tech" PRO Canon would eat my $1100 D7000 for lunch and I would feel great about this purchase, but I don't.
Bottom line if I'm reviewing the camera - Yes, my 5d3 takes nice pictures. Yes, the autofocus is fast and accurate Yes, I do get better low light capability than my D7000(1.5 stops better) and from what I've read about 1 stop better than the 5d2 and 2 stops better than the 7D. The 5d3 has very good quality up to 3200 and usable up to 12,800.
For me...it's $3500 for better low light and slightly better IQ.....I'm going to have to really think about this.