I use this lens for high school football, softball, baseball, and soccer. It has its pros and cons.
Pros: It is a zoom. When the guys with the white 300 f 2.8 back up into the parking lot to get a shot from the end zone, I just dial the Sigma back. Its also about $1,200 cheaper than the Canon. I find the IQ is very good - the Canon may be sharper and the contrast is not as good as the Canon, but you would be hard pressed to detect much difference in real world conditions. It seems solidly built. I've had mine for two years with no problems.
Cons: It doesn't focus as quickly as the Canon. I know I miss a few shots, but I also get more than enough good shots to sell. It isn't weather sealed as the Canon is, so a rainy night means bringing along the plastic garbage bag. There is no IS on it, but I don't need it for sports.
Canon 1D MkIII, manual mode, f 2.8, shutter speed 1/500, ISO 2000, no exposure comp, 300mm focal length. Taken indoors at the Alamodome and is heavily cropped.