Since it hasn't been mentioned in here, I own the Sigma 12-24mm f/4.5-5.6 AF II DG HSM, and think it performs very well as an ultra-wide. It does suffer the usual ultra-wide disadvantages -- distortion, vignetting, chromatic aberration (I find the CA to be worse on this lens than others), and it also won't take a filter on a FF body. But it keeps lines very straight (great for architecture and landscape photography) and there's very little pin-cushioning. However, the primary advantage of this lens was that it's the widest non-fisheye full frame lens. When I purchased it, I owned a 7D but had dreams of upgrading to a 5D and didn't want to dominate my lineup with crop lenses. After upgrading to the 5D, this lens functions great!
It's a little pricier than the EF-S lenses in the ultra-wide category, but if you purchase it and eventually upgrade to a FF body, you won't have to worry about dumping it for a FF lens.
Overall, I've been very happy with the Sigma 12-24mm, and it also fits perfectly with my 24-70mm and 70-200mm.