I upgraded from the 7D to the 6D. The 7D is still great, but I needed high ISO performance for events, and the 6D does not disappoint. In low light, it basically becomes a 1 point AF DSLR, but that one point grabs focus like a tractor beam . For my style, FF is the way to go.
What about the AF
I have both a 7D and 6D. The 7D autofocus is of course better, but I find the 6D AF good enough for 95% of the things I shoot. That said, I don't do much wildlife or sports photography, mostly kids/family, landscapes and event photography. The 6D AF is fine for almost everything except for fast and erratically moving subjects.
I just bought a 6D, keeping my 7D which I've used for a couple years. I still love my 7D and will use it for rapid focus and extra reach. I also think at 800 and under ISO the images are fine. What I've found with the 6D (only a couple weeks use) is that the ISO performance is amazing. At 6400 the noise is probably equivalent to 800 on the 7D, but the noise is not as unattractive - not sure what the difference is...
The 6D seems to produce a richer image color wise as well.
I love both cameras!