Great stills, not so great video
Gizmodo has reviewed the EOS 6D and found it to be a great still camera. However, they did notice the video produced by the EOS 6D is prone to pretty bad moire, something that doesn't affect the EOS 5D Mark III.
As I've told most people, if serious video is your thing, get a 5D Mark III. We are hoping the moire issue can be resolved somewhat with a future firmware update.
“If you are most interested in stills and don't care that much about video quality, the 6D is a great value—it produces top-notch pictures, with very few compromises, at a lower price than we've ever seen. But if you are looking for the video prowess of the 5D Mark III, plan to pony up that extra $900.”