I'm going to go against the grain a little here...I recently purchased and then sold a Sigma 10-20, not because it's a bad lens - it's very good, more for the reason that I didn't find it as useful as expected for landscape shots.
The reason being that because scenery becomes distant you need to ensure you have a very strong foreground element, although this doesn't sound a bad thing I just found for my preferred "style" of landscapes the foreground element was becoming the main focus of the image, not really the type of shot I prefer. Hopefully this makes sense :shrug:
Most of my shots are between 17 - 25mm, anything wider I now shoot as a Pano
[edit] Just to add that UWA lenses tend to exhibit distortion - some find this "effect" adds to the shot, but personally I didn't like it
Simon