I've had a 5D 2 since launch, (sold my Nikon D300 for it) and I gotta say, with my Nikon I was nailing 90% of my focus. With my 5D 2, about 50%. It's leaves me with so little confidence in nailing focus that I have to take the same image 2-3 times again, just to make sure I'll get one with good focus.
So why would anyone take the 5D 2 over the 6D? 5D 2 has 1% more view finder coverage, and has better ergonomics. Oh yeah, and CF
Everything else, 6D is at least as good, and some parts better. Better low light performance, same AF, better video, better weather sealing, a newer processor, and a better rear LCD (900k vs. 1.2M)
One thing I loved about my Nikon was that it was compatible with every single piece of nikon glass ever made, no adapter required! And it metered too! But yeah, the D300 was more of a Pro-Camera than my 5D2, with all things considered in regards to construction, focus, view finder, etc.