No complaints with the 5D mk III. I've taken about 3500 shots with it, some things I've noticed:
The AF is excellent. Focusing is extremely fast and the camera does a great job in the dark.
High ISO has very little noise. I was using the 7D previously and would notice sensor noise at anything above 1,000. Shots from ISOs higher than 3200 were pretty unusable. The new 5D can be pushed to 6400 or higher. I took this a few days ago at ISO 5000.
Sorry I don't have a larger version accessible.The image is a Jpeg so there was extra noise reduction performed in camera. I like that the camera can record to both CF and SD, I shoot raws to one card and jpegs to another.
The FPS are a step down from my 7D but I don't feel much of a difference. The silent shooting mode is really nice, almost inaudible at a few feet.
I haven't even messed with the video but the camera has some new features from previous canons.
I have no experience with the D800 so I'll let someone who has used one weigh in with their thoughts. I would recommend the 5D mk III, overall it has been a great buy. Do you have a significant investment in glass? Might affect your decision? What sort of things do you like shooting?