Received my 5D3 yesterday, and quickly fired off a few frames. In a dimly lit room, with camera on full auto. Using my existing 24-105L, which has yet to be micro adjusted to the new body.
I pointed the camera at the dog and clicked. The camera chose ISO 12800, F5.6, 1/50. OMG. There was very little light in the room but the camera focused INSTANTLY. The in camera preview looked good, but looking at the RAW file in Lr4.1 showed that 5D3 captured detail that I did not see in the room. I was reluctant to shoot over ISO 1600 with my 7D, but this camera is a game changer.
A few hundred frames later, after shooting in a variety of settings today, I can tell anybody who is naysaying the 5DMkIII the following:
1. You haven't shot with it. If you did, you would have nothing to cry about.
2. The "not enough of a change from 5D2" crowd have definitely not shot with it.
3. The AF is so good it becomes transparent to the photographer. You forgot about it. It happens FAST. 5D3 isn't just for weddings. You can use this body for anything.
4. Exposure is dead on accurate.
5. In camera HDR really works well. Merges three exposures and keeps the originals around if you want them. Best of all worlds.
6. Ergonomics are the best Canon has ever done.
7. Quiet is good. 5D3 is QUIET.
8. Moving the old hard-to-find DOF Preview button from your left hand (thumb) to your right hand is intuitive and makes it possible to check depth of field with one hand.
9. SD cards let Mac users cut the cord!
10. 22MP is big enough to print big (I routinely print 30 x 40) while keeping file sizes manageable.