OK, a preface. I have a 17-85 USM, a 10-20 HSM, and a 70-200 HSM. I COUNT on the 70-200 for 99% of my race pictures, where AF speed is critical. I use AI-SERVO so that the car remains in focus as it gets nearer to me, etc. Last night, I attempted to use my 70-200 at a martial arts event. I had always thought HSM was an ok thing, and comparable to USM. However, the AF speed was too slow, so I ended up using my 17-85, because of the nice AF speed. I would have liked the extra zoom, but it takes a split second to even try to focus with the HSM lens. Then, I thought.....maybe it's just that focusing takes longer on a longer lens....so I tried my HSM 10-20...which also had the slight focus delay, that my USM doesn't have. Anyway...if the Canon 70-200 f/2.8 (IS) USM lens focuses faster than the Sigma 70-200 f/2.8 USM.............then I get it, so I can have more accurate shots. SO... does anyone have experience comparing the AF SPEED/RESPONSE on these two lenses?