Today, I put the 5dmkIII through some tentative testing at a wedding
I don't feel qualified to give a full technical review so instead, I might just give you a few gut feel thoughts about how it felt.

- The ergonomics are great - as silly as it sounds, the dial turns, button pushes all feel much 'softer' than the 5dmkII
- It feels and handles like a 7D which is not a bad thing
- You can literally customise the sh@#$t out of this camera for C1 C2 C3 mode - I set C1 to auto iso, set AF parameters, set AF drive to continuous shooting, set minimum shutter at 1/80, -1/3EV exposure bias and when stepping out from the dark church into bright sunlight following the groom, managed to get some half decent shots before going back to full manual
- The mode lock button is awesome - I don't start shooting and wonder why I'm all of a sudden shooting in TV mode instead of manual
- The LCD seems an improvement over the mkII
- High iso shooting - well , I'm sitting here very chuffed at the moment - I was worried that the reports of great iso performance were all due to the algorithms for noise reduction in camera for jpegs but the RAWs at 3200 look pretty damned good! I didn't push 6400 today but will definitely do so next time (ie. when I've owned the camera for more than 1 day lol)
- PS. this isn't a compare with nikon thread - I'm just giving an impression that for the camera I was replacing it to shoot weddings with (7d) it is a huge improvement - Marianne shot the entire day with the 5dmkII and had no issues of course. I'll take it out over the next few weeks for landscapes but I honestly don't expect much difference from the mkII except for star shots

here's a couple of examples without editing from the RAWs out of lightroom to jpeg unsharpened
1600 iso
1600March1321.jpg

3200 iso
3200March1104.jpg