Aside from the better quality of the EF-S 10-22mm lens, it offers a significantly wider angle of view. So much so that at 10mm, there is a fair amount of distortion. An ultra wide angle can be an extremely powerful tool, particularly when you want to get really close to a foreground subject, and still capture great breadth and depth to your scene. The 18mm wide end of your kit lens can't hold a stick to the 10mm wide end of the 10-20mm EF-S.

A simple comparison can be made between these lenses and their EF (full frame) counterparts, as well. Sometimes that can be helpful in making a decision. Since Canon entry-level DSLR's use APS-C 1.6x crop sensors, it is easy to calculate the full-frame equivalents. The 18-55mm EF-S kit lens behaves like a 28-88mm EF lens. The standard EF counterpart would be the superb 24-70mm L series lens. Similarly, the 10-22mm EF-S lens behaves like a 16-35mm EF lens. The direct EF counterpart for that would be the 16-35mm L series lens. Canon's EF 16-35mm f/2.8 L II USM is one of their widest zoom lenses, with the only wider one being the new EF 8-15mm f/4 L USM FishEye lens. That should put the EF-S 10-22mm lens in perspective.

Here are some links to photos that make use of ultra wide angles to get extremely close to their foreground, while still encompassing great scene breadth and depth. I think the last link in this sequence particularly demonstrates the power of wide angle:

• Twilight Floating Memories: 16mm/APS-C
  • This is a little beyond the limit of your 18-55, for reference
• Down in Sinkhole Land III: 10mm/APS-C
• High Tatras National Park: 17mm/Full Frame
• A Small Oasis: 10mm/APS-C
• A Break in the Storm: 10mm/APS-C
• Korpudalur Sunset: 10mm/APS-C