I have had this lens for just about 1 year now. Initially, I thought the image quality was great at times, not so great at others. I had infrequent problems with my Canon 20D NOT recognizing the lens, The display would seize up when the lens was not recognized.
I finally sent it in for warranty repair and boy what a difference. The logic board was replaced and the lens was recalibrated. I can see the difference in the quality of the images. I expected this good a prime lens when I first bought it, but I'm glad I'm able to experience it now.
For my price range, this lens does very well. I'm very much a parental novice (kids sports events, nature, etc), so I've only taken maybe a few thousand pics with the lens. But I've been very happy with it, especially after the warranty service.
For future Sigma purchases, I will probably only go with the EX series just to get the longer warranty. I'm a little gun-shy about buying another non-EX lens after this experience.
I didn't rate it higher only because it required repairs out of the box. I would've given it a "9" otherwise.