I've always been pleased with my Sigma lenses. In this 70-200mm focal length, I do own the Canon L lens, which is very nice, but also expensive and heavy. It is my favorite "zoo" lens, but truthfully it would be nice having a little farther reach. A 200mm on a digital camera (200mm X 1.6 = 320mm) may be perfect. The real Canon lens (and I'm guessing the same would be true for a Nikon), does focus extremely fast. However I've never really been discouraged by the Sigma's focusing maybe a little slower. Depending on the cost difference, the Sigma could be a very nice buy.