I jumped at the chance to have an f/2.8 lens at 17mm, but it's just too soft at wide apertures. I did some back to back comparison at 28mm with my Canon 28-135 IS lens, and the non-L Canon lens was sharper. Not too happy about that.
I have had good luck with all Sigma lenses, except this one. Maybe this copy just isn't that sharp, but from what I've read, maybe not. Right now, I think I'll save some $ and switch to the Canon 17-40L f/4.