I have both and my rather random thoughts would as follows:
The 6D is a really good fit in hand, better than the 5D/5DII. Although only slightly smaller it is more 'handy'.
The AF seems to be more precise than the 5D/5DII on all points.
The 6D uses some kind of colour metering whereas the 5DII doesn't. Haven't made my mind up on this yet, it behaves differently to the 5DII in some situation ( bright blue).
The overall 'IQ' is remarkable. There is a subtle improvement in low ISO, very good, film like tonal graduation and greater latitude at both ends of the EVA range. High ISO is astonishing and way ahead of the 5DII from about ISO 800 onwards. Overall I would put the 6D up against an A7r any day, there is not as much practical difference between 20 mp and 36 mp on FF as you may think, and the whole DR range on the 6D is very useable, making real world differences academic most of the time.
The 6D has a polymer top plate which feels identical to the mag alloy top of the 5DII but does get scuffed and scratched more easily as it is relatively soft.
Now the bad bits, at least for me. It uses the 'second tier' userface and ergonomics. The controls work fine per se, but moving between each is annoying, at least for me. However as you have sold your 5DII you won't have this issue. The top command wheel is very softly click stopped, and the rear command wheel is, well to be polite, disappointing after the 5DII. Also the flash compensation has to be done via the main menu although you can assign it to quick menu and then it is quite useable to be fair. So to summarise I'd say it's not as 'positive' on the controls as the 5DII series, but you get used to it and then it works fine.
Thanks! This is great info about the ergonomics. Flash compensation is something I rarely use nowadays, I mainly work in manual flash so it's not an issue. I did notice a slightly less robust feel in the camera shop but the MKII was a brute and it shouldn't be a surprise that the 6D is not as positive as you say in that respect.
I don't really need a rugged camera but as long as it holds up in everyday use I'm OK with that.
I noticed more and more that my camera stayed in M mode and I don't think I ever used the quick menu on it. As long as I can change shutter speed and aperture the same as all Canon cameras I'll be happy!
How's the scroll and pan speed when when you zoom into the preview? The 5DII was slow, as was the 7D before they released the v2 firmware. Does the 6D have that issue or is it running like a greased 7D pig?