After carefully reading the reviews here, and trying to decide between the Sigma 10-20 and the Canon EF-S 10-22, I purchased this lens. Initially I was very impressed. But although very sharp, and of good build quality, it developed major problems, which resulted in me returning it, and switching to the Canon EF-S 10-22.
Sharpness - excellent. After all the negative reports here I carefully compared my Sigma 10-20 against the nearest lens I had in terms of focal length, my Canon f/1.8 28mm prime. Photos taken at f/8 with the Sigma were sharper in the centre, both at 10mm and 20mm than the prime. At the edges the prime had the edge, but only just.
Focal range - fantastic. In old medieval towns pictures of buildings, streets, squares, piazzas are easy and fun to take. A sense of space can be captured with this lens making it excellent for both landscapes and interiors. I think the extra 2mm at the wide end is important for all of this. 12mm would be too short.
Build - Very solid. The focus and zoom rings are nice and tight - better than my Canon f/1.8 28mm and f/1.4 50mm primes.
Lens hood - Included in the price. I actually found it to be quite good and locks in place.
Packaging - environmentally friendly. No styrofoam in sight. The lens comes packaged in its own case, which is padded at both ends. Nice solution. Canon, take note.
Price: significantly cheaper than the Canon EF-S 10-22.
Size/weight: Large enough for my left hand to get a good grip without having a finger or two past the hood, yet small and light enough to be manageable.
Lens cap. Unusable without taking off the lens hood first. Solution: the LC-77 Nikon centre-pinch lens cap works nicely (and you get to have Canon, Sigma and Nikon logos on one camera ...)
Filters: Slim filters caused vignetting at 10mm in the top left hand corner *only*. All other corners showed no vignetting. Others here have reported this. It's very annoying - and difficult to spot through the viewfinder.
One day in summer I went out to take photos of a town on a lake - and right in the middle of shooting the camera suddenly lost AF, and the viewfinder went very dim. Investigation showed the lens aperture blades had stuck somewhere around f/16 or f/22. After an hour or so of fiddling around they released. For several weeks the problem didn't return.
Then, on another trip it happened again. Once the blades jammed, I lost all chance of taking the shots I wanted. Pressing the DOF Preview button sometimes released the blades after a few minutes, sometimes it didn't. This problem continued to repeat itself on every sunny day. My Canon lenses had no such problems. None of the days were particularly hot, some not even warm.
After much experiemntation I concluded that the lens was prone to jamming at any temperature over 10.deg C when the sun was out.
I have now returned the lens, and switched to the Canon 10-22 ...