Just got mine... It is amazing! It's so responsive and easy to use. The autofocus is ridiculously fast, even in low light. The viewfinder is huge, and the image quality is incredible. The first night I got it I exclusively shot at 12800 ISO inside in low light. Noise is visible when zoomed in, but non at all obtrusive. I seriously think that 12800 on the 5D Mark III is a bit better than 1600 on the T1i! So for everything other than landscapes on a tripod, I've left it on Auto ISO in aperture priority mode.
The noise performance is amazing, but I kind of expected that (based on early reviews). What I did not AT ALL expect, however, was the overall sharpness and clarity of the photos. I have to say this was what surprised me the most. I shot everything in raw, and then converted the raws to jpegs in camera with no sharpness applied. When you zoom into 100% it still looks like a normally sharp photo! You can barely see any difference in sharpness between that magnification and zoomed back out. - I still can't believe it.
Also, the feel of the camera is great too. It's really ergonomic and just feels right in my hands.
It's hard to describe how good this camera is. I think that when you put all of these great features (like 61 point autofocus, 6fps, noise performance, image sharpness etc...) together into this one body, the camera becomes really special. All these features work together to make the camera way better than what you see on the spec list.
Another thing adding to this surreal experience is the two L lenses that I bought with the 5D. The 24-105 f4 and the 16-35 f2.8 II. I don't care that the 24-105 is only f4 and not 2.8 because the ISO easily makes up for that lost stop on the 5D. The background blur is also easily achievable given the compression at 105mm and nature of a full frame sensor. The 16-35 is great too. It's equivalent to what my 10-22 was giving me on my T1i, but it's 2.8 which makes it even nicer.
Both of these lenses are built amazingly and are a huge jump up in terms of build quality from the 17-55 f2.8 and the 10-22. The focus and zoom rings are so smooth and they feel like they can withstand some abuse while shooting.
I can't wait to go out and keep shooting with all this amazing new gear!
Just took this shot with the 5D Mark III and the 16-35 II tonight:http://www.JordanSternPhotography.com