The 10-22mm is Canon's best AP-S lens, in my opinion. The quality is up there with my L series lenses. There's a new 10-18 that's less expensive, but I haven't tried it. 

Ultra-wide lenses are a lot of fun. I like the way they distort much more than fisheye lenses. Of course, if you're even slightly off in an architectural setting , it'll doom your photo lol