Plain and simple this lens is a contender! It's commonly compared to the Tamron 17-50mm and Canon 17-55mm. The other lens it is sometimes compared to is the Canon 17-40mm L. If you're shooting on a full frame sensor then you're only real choice here is the L lens...And it's a great lens. I don't own it but I've shot with it before.
However for those with cropped sensors...I'm telling ya, get this lens. If you're on a tight budget, get the Tamron. However, I'd probably just simply stay away from the Canon 17-55mm because I think it's overpriced. If it was an L series with image stabilization...Well, then maybe. Maybe it'd also cost more.
There are a TON of reviews on this lens, so read here and Google it up. I won't repeat what others have said.
However one thing I did not know when buying this lens.. This lens has the new Sigma finish. It does not have that "crinkle" finish. You know that kinda visually textured/sandy matte dull finish (which was pretty smooth to the touch but not glossy smooth). I actually liked the old finish...But many people did not because it could peel off with a lot of wear and tear. I never had that problem, but it's worth noting because you actually don't read about it much. I know it has nothing to do with image quality, but don't be surprised when you pull this EX lens out of the bag for the first time and it has a smooth plastic finish.