Upload & Sell: On
I recently bought a 6D, to complement my 7D, which I have used a lot for shooting youth sports and birds. The primary purpose for the 6D is landscape/portrait. I view is as a complement rather than a replacement for the 7D.
That said, I took it to shoot a CYO basketball game in the worst-lit gym in Marin County to see how it compared to the 7D. I was favorably impressed. Grandted, the overall operating speed of the camera was pokey compared to the 7D, both in terms of frame rate and speed writing to the card. I would not choose the 6D if shooting sports were my main use for the body.
But the AF worked flawlessley, using the center point, which is what I always used on the 7D for this purpose. The high ISO performance (ISO 6400) was probably about a stop to a stop and a half cleaner than the 7D. Being able to use ISO 6400 and get acceptably clean images enabled me to use my 70-200 f/2.8 in this gym, which focuses brilliantly, rather than my 50mm 1.4, which was my primary tool on the 7D in these situations, which was okay, but slower to focus and where the narrow DOF at wide apertures left less margin for focus error.
I found the biggest challenge was keeping the focus point on the subject. The selected focus point is not as clearly prior to the shutter being depressed as it is in the 7D, so it was easy to lose track of where the focus point was, exactly.
The overall keeper rate and image quality with the 7D and the 70-200 was measurably better than I had been able to get on the 7D, either with the 70-200 (too noisy) or the 50mm (limiting and unforgiving).
In sum, if you are considering this body as an occasional indoor action tool, while it's not a perfect body for that purpose, I think you will find its AF more than acceptable
Canon EOS 6D EF70-200mm f/2.8L IS II USM lens 200mm f/2.8 1/400s 6400 ISO +0.3 EV