Generally, I've had better experience with Tamron, than Sigma, but Sigma has gotten much better recently, releasing more and more lenses that challenge the best from the OEM's. I ended up picking up a 70-200 OS and the performance is very, very good. It focuses fast. It is sharp wide open at the center where it counts (generally for a long focal length lens the center is more important.) The only quirk I've encountered with the Sigma is it refusing to focus as close as it should be able to. In other words, up really close, in the last few inches of minimum focus length it will hunt for focus and give up...not racking all the way. However, I can manually move it into focus and then it happily stays there. I haven't checked to see if this distance is inside the advertised minimum range of the lens. If it is, I guess I should consider it a macro "bonus."