I've been able to shoot Australian Rules Footy - which is arguably one of the more unpredictable sports in terms of ball movement, and undoubtedly one of the biggest playing fields -with both my K10 and more recently my K5. AF-C isn't great to say the least, especially on the '10, so I'm often looking for work-arounds. I probably shoot more in AF-S, tbh, but thats as much because it's difficult to keep the focus point on the desired player as much as irregular predictive patterns from the AF system. In any event, I can get results that many people tell me are excellent. Including AFL club websites:
I bought a 7D as well, used it for a while, decided it's much-vaunted AF system wasn't offering a huge step up relative to the K5, and so i sold it. Had i had a tonne of Canon glass, the decision would have been reversed. But I love the dynamic range and colors of the Pentax, so maybe the Canon was always doomed.
My biggest gripe is the scarcity of long, fast glass for sporting use. Sigma is our saviour, but even they won't mount their 120-300 2.8 in Pentax. So i use the 70-200, and the 150-500mm. Then use the 5's ISO ability to make the long siggy work. Something the 7D couldn't do.......
swings and roundabouts, boys and girls, i'm afraid, unless funds are unlimited. The pro's use Nikon and Canon for sports for several reasons. but unless you can afford the specific models, everything else will require a compromise somewhere along the line. My photography of sports is good or bad because i'm good or bad. I've stopped worrying about the camera arms race.