Just got my 6D in the mail today. Replaced my 5D2 (sold for $1500, which is about what I paid for it earlier this year). I intended to get the 5D3 but decided to go with the smaller/lighter 6D and save some money in the process. I've only had it for a few hours but I can tell you that the 6D is better than the 5D2, hands down. It may not be a HUGE improvement, but there's nothing about the 6D that is worse than the 5D2.
Out of the 5D2, 5D3, and 6D, the 5D2 is the worst camera! It's a great DSLR, but if you could get the 5D2 and the 6D at the same or a similar price there's no reason not to go with the new body.
I would absolutely love to have the AF of the 5D3 (or even the 7D!), but other than that I can't really find fault with the 6D. Performs great in low light (both AF and ISO). I was always hesitant to go over 1600 ISO with the 5D2, with the 6D ISO 6400 is no problem. Even the shots at 8000 ISO I would consider useable.
6D is a great camera. I know the day will come when I sell it and replace it with a 5D3, but until then I will enjoy it immensely!