(First; excuse me for my english but i am from Spain it is not my language).
For me it is not a bad lens. Compared wit 17-85 USM IS at 20 mm this is much better in all cases. The same compared with Sigma 12-24 at 20 mm also.
I use this lense for a special moments that i need more precison that give me the other mentioned lenses, or with low ligth.