I sold the Canon kit-linse 18-55 and bought a Canon 24-105 IS. Thus I needed something more wide, and while 10-20, 12-24 or so would have been nice it was a bit to expensive at that time.
This linse is really perfect in most ways. It's extremely sharp compared to the price and with great contrast - well most Canon L have better contrast, but they dont come for $220.
If you cant afford a zoom in the 1x-2x area and dont want a fisheye, this one is perfect.
Its used on a 20D so I cant say how it would FF, but on a 1.6x nothing negative can be said.
Well, one little negative thing. On AF its not USM and not the fastest fox in the forrest. But its a wide, so no problemo. Switching to MF is done by first swithing a switch (like normal) to MF and then "pulling" the focus ring when its placed on a certain spot. Since its a wide linse and not a tele I dont see it as a big problem, but this part could be improved.