"Check out the Canon EF-S 10-22mm f/3.5-4.5 USM Lens. First, before you buy. It was designed exactly for your camera. The Siggy was not."
I agree with that opinion. I'm from the school of thought that says, "Buy Canon Lenses, first. Buy third party only when Canon doesn't offer a comparable product."
The EF-S 10-22mm has an effective focal length of 16-35mm on an APS-C body. Canon offers "L" series lenses at that focal range. I own both the 10-22mm and a 16-35mm lenses, and the 10-22mm is the best EF-S series lens that I have used.
While the EF-S 10-22mm might lack Image Stabilization, it more than makes up for it in speed. You can still get f/4 from the lens at 18mm. I have no complaints about the image quality with the lens. The above shot was taken at 20mm from a inaccurately leveled tripod. The image is quite rectilinear, and shows little signs of keystoning, which is why I took the shot in the first place....to test the lens' performance.
The contrast and color from the lens is excellent. It focusies quickly and quietly. I would compare this EF-S lens against the EF 24-105mm f/4L IS USM for Image Quality at its' short focal lengths. In fact, I think the EFS 10-22mm has less barrel distortion at comparable angles-of-view than the EF 24-105mm f/4L.
