The Canon EF-S 10-22mm f/3.5-4.5 USM Lens' focal length range fits nicely below Canon's popular 24/28-XXmm general purpose lenses such as the
Canon EF 24-70 f/2.8 L Lens. These two lenses (or other similar combinations) could satisfy all focal length needs below 70mm for many people. Even if your main lens is 16/17-XXmm, the 10-22mm is much wider and would make a useful addition to your kit.
In my opinion, shooting at 10mm requires more skill on the part of the photographer than shooting with longer lenses. The Canon EF-S 10-22mm f/3.5-4.5 USM Lens is even wider than the
Canon EF 15mm Fisheye Lens on the same camera. Composition at this focal length becomes more difficult. As this angle of view takes in a lot of
background, care must be taken to make it look good - it probably will not be blurred out of focus. Watch out for your shoes and shadow - they are easy to include in the 5% of the picture missing from the current EF-S DSLR's 95% viewfinders.
Close-up perspective at 10mm can result in wild and dramatic photos. This angle of view opens up a great opportunity for creativity. But if you want parallel lines to remain that way, you must shoot them from an exactly perpendicular angle (or fix them later during post-processing). This perspective distortion is up to photographer to control or dramatize as desired. Architectural photography is one 10-22mm use with many parallel lines. Interior photography is another similar good use for this lens.
While there is no law against taking pictures of people with the Canon EF-S 10-22mm f/3.5-4.5 USM Lens, they better be self-secure, fun-loving and smaller than you if they are going to see the results from close shots! You can make monsters out of your angels. Everything that sticks out will be greatly enlarged - especially noses.