Like most people, I started a long time ago with cheap lenses, cheap zoom and got tired of having a x Megapixel camera that actually sharply resolves less than half of that.
I decided I was ready to trade off some versatility in order to gain a wider angle. I bought the 10-22 and never regretted it. That lens is amazing, excellent quality, it feels solid, especially for a EF-S lens (i.e. the typical 18-55s look like a cheap plastic junk compared to that lens), it will distort at 10mm but not terribly, if it annoys, you can undistort it easily with all the available software out there.
When I go on a trip, I always bring it and use it as my main lens. the only drawback is it's a little heavy for long walks (but nothing compared to my 100-400L), it's worth it!
I love this lens so much, I will need to make a move from my 20D to either the 7D or 5DmkII, and the only reason I am hesitating is not only for the price difference from the 7D to the 5D, but having to give away that lens and pay a different to get a 17-40mm which I didn't see much difference (and losing equivalent 1mm (1.6x crop)). The other 16mm (equivalent) is just too expensive once you tried the 10-22.