I bought this lens to shoot softball games, so I was frequently using the lens at 200-300 to get closer to the action on the field. Shooting action late in the day, I had my 10D set to ISO 800 and 1600 to get reasonable shutter speeds. The lens was wide open, unfortunately. I kept the camera on a tripod to eliminate shaking. Results were disappointing -- very soft. I had to apply aggressive sharpening to rescue the images. This lens is not a good choice for action sports. If I were going to do sports on a regular basis, I would invest in one of the fast L-quality lenses.
If your subject allows you to shoot on a tripod, stopped down to f/11 or greater, results are better, but still not great. Not recommended.