Recently returned from spending some time in Israel. I found a well-priced 10-20 F4-5.6 in the marketplace shortly before I left, and I couldn't resist. I am so glad I brought it along! I used the Pentax DA 18-135 on my K-x 80% of the time, but almost all the rest of the time I had the 10-20 on. I'm still learning how to use it well. (Closer, get closer!) If I position the camera correctly when I take the shot, it really does a great job of keeping things rectilinear. I need to play w/ it more to artistically use it non-rectilinearly.
I've attached one example I would never have gotten without it that shows off it UWA ability. It's in the Bell Caves at Bet Guvrin. It was actually rather dark, and this is a handheld shot.