10-22 EF-S Lense

What do you guys think of the Canon 10-22 mm EF-S Lense? I have seen them a lot on Canon EOS 20D's for some reason.

Well, for one reason, it's a very good lens.

The 10-22, after factoring in the 1.6X crop factor, gives an equivalent focal length of 16mm to 35mm, which is a very usable range. If you couple that with the 17-85, you have yourself covered all the way from 16-135mm (effective).

There are other lenses that come close to the 10mm, like the Sigma 12-24. The problem is that they are full-frame compatible lenses, while the 10-22 is an EF-S lens and can be built smaller and lighter without the need to fit on anything other than SLRs with the APS-C sensor (20D, Rebel XT, etc.)

A 10-22 full-frame zoom lens would be a relatively large, heavy lens, and would likely cost a lot more money.