Hi Andrew,
We use one of these as our main lens and would estimate that at least 80% of our photos are taken with it. Check out the links in our sig for some examples - we shoot loads of different sports with it - indoors and out. I think that all of the shots in our introductory thread were taken with this lens. We have used it on the 20D, 30D, 40D and currently the 1D MKIII. Needless to say, we love it!
Most threads we have read about this lens usually compare it to the Canon/Nikon f/2.8 300mm primes. However, in our opinion that is not really a fair comparison.
Firstly - you would expect any prime lens to focus faster than a zoom lens at the same focal length - that is surely the main advantage (for sports shooting) brought about by the simpler optics.
Secondly - nobody ever seems to make the point that this is the only zoom lens you can buy that goes up to 300mm at f/2.8. The flexibility has always been the deal breaker for us. In a sports setting, especially outdoors on a pitch, it simply gives us the most opportunities to take pictures of the action, which is what you we there to do after all.
We have never once envied the guys with a 300mm prime on one body and a 70-200mm on another. We are totally confident that the AF on our lens works faster than having to switch between cameras
Anyway - those are our opinions. We are happy with the AF and, most importantly, with the results the lens gives us.
Sam & David