I have shot Football, soccer and basketball with this lens and it has done me proud.
All were on the 400D (havnt tried with the 1DmkII yet)
Focus is a bit slow at times, to me the only slow bit is if it reaches infinity and has to come back to something close to focus on.
Once you know how to use this lens, the focus isn't that bad! It's no 70-200L but its under 1/2 the price!