Ooh I loved the 10-20 back in using Nikon. I've still yet to get an UWA in Pentax. Still crossing fingers for an 11-16, and I'm pretty sure to get a DA15 someday.
Anyway on using it, it's really fun at first but it's quite hard to shoot efficiently with it. Few things you can do:
(1) Push in TIGHT. Go close. Real close. You're bound to get something interesting.
(2) When capturing a scene you need (A) foreground and (B) background. These 2 combined can make a really good shot. Perfect example is axl's photo with the guy on the street.
(3) Play with the relationship of the subject with the environment. The really cool thing about UWA lenses is that it can make something look REAL far away, you can add some real depth in a photo. Here are some examples:
(4) Don't be afraid to take portraits too! You can get some really interesting and unique ones with UWA lenses. Just be mindful of your angles to control the distortion on the subject