I have a 70-200 Sigma zoom lens for my Sony A100. Shooting a sports event was the first time I have really used this lens, and I must say that I am not impressed with it. It produces soft images even with a still subject.
I changed over to my trusty Tamron 90mm lens, and I was able to get some decent sports photos with it - although I would have liked to be able to get closer to the action as with the zoom lens.
By the time the next sports season rolls around, I would like to have purchased a Minolta 70-210 f4 zoom lens. From all the reviews I have read, this old-timer has great glass for sharp images - one of the higher quality lens next to investing thousands of dollars in the APO or Zeiss glass.
Looking forward to the next spring Lacrosse season to capture some better images. Perhaps a few football games this fall if I can save up for the new lens.