I tested 6D a year ago. I did some shooting in very dark club (ISO 12 800, F2,8, 1/60 s) and on the evening street and was quite happy with the AF performance. Most of the time I used central point, outer points were significantly slower, but still accurate. As a 1D Mark IV owner (at that time) I was really positively surprised as one shots AF was noticeable better.
As a sport photographer I also give it a try in indoor cycling under the poor lighting conditions (ISO 6400, F2,8, 1/320 s). Central point was working acceptably well and was able to focus on moving cyclist. Outer points were no use at all. 1D Mark IV performed significantly better under these really hard conditions.
For me only shame was a lack of dual memory slot, otherwise it'd be perfect second body for weddings, parties, proms etc.