Originally Posted by cgust

Ive seen quite a bit of complaints on the net about sigma's quality control but I've had great luck with them so far. I also think the amount of complaints has dropped significantly over the last few years and sigma is becoming a big competitor. Comparing my sigma 120-300/2.8OS to the nikon 300/2.8VRI... the AF speed is faster in the nikon (which im assuming is the same speed as canon) but the sigma is no slouch and I can track birds in flight just fine. I cant think of a reason I'd need it any faster. Plus the sigma has to move much more glass because it is a zoom lens. Sharpness goes to my sigma though. Its definitely the sharpest lens ive owned and Ive had my share of good nikon glass. So...Id recommend sigma for sure! Before buying just be sure to read reviews and lean toward the EX versions or their new Art and Sport models.