Another fun little lens, the 10-22mm lens is quite an old design and while there have been a lot of ultra-wide zooms released recently the Canon is still highly regarded.
Giving an effective 35mm-equivalent focal length of 16-35mm this was bought to complement our general purpose zooms (initially the 17-85mm, now 15-85mm) where they just aren't wide enough eg big landscapes or cramped rooms.
For such a short focal length the lens is surprisingly long but very light. It is well built although the focusing ring could be a little bigger. We tend to keep most lenses with a filter on the front element, either UV or circular polariser, with slim filters we see no extra vignetting. The lens internally zooms and focuses, which is a nice feature with filters on. The optional hood is very shallow but every little helps at these ultra-wide focal lengths.
For a very wide zoom lens distortion is handled well, certainly a big improvement over kit lens zooms, something that can be very obvious sometimes when used for architecture. The only noticeable distortion is the natural distortion that an ultra-wide rectilinear lens displays, and gives the lens much of its fun and creativity factor.
The only real downside to this lens (and a lot of the other ultra-wide EF-S compatible lenses) is being limited to APS-C cameras. Having recently acquired a 1Dmark4 we're stuck with having no wide lenses to use on it. Luckily this isn't a big deal as the 1D is mostly for birds and macro, the 500D is more than enough camera for landscapes.
In the field I'm always torn between using the 15-85mm with its much greater focal length range and reasonably wide short end, and the 10-22mm with its limited range but extreme field of view. Often the 15-85mm isn't quite wide enough and 10-22mm not quite long enough so we end up switching lenses (and filters) a lot. Unfortunately there's not much to be done about this (other than be more disciplined with our composition and positioning), a zoom that covered all of these lengths would be either horribly expensive or have far too many optical compromises. This is where multiple bodies is the only real answer and luckily we can usually do that.
The lens doesn't get used as much as it should; while ultra-wide angle can give very striking results it is quite hard to find scenes that fill all that field of view or don't look weird with all of the wide angle distortion. Again, maybe we just need to better our compositional skills.