When I first got my Sigma 10-20, obviously the first thing I did, out of excitement was clamp it onto the front of the camera and have a look around. You only really appreciate it though, when you do some comparisons with your other lenses.
Here is my bathroom at 10mm on the 450D:
Now 18mm:
and finally 35mm, which is a pretty standard wideangle for most compacts:
When you compare the 35mm and 10mm images it really brings it home just how wide 10 is... for me the Sigma is great for indoor photography and landscapes. The perspective distortion of the lens also makes any room look massive.
I did the same test on my cactus (sorry for the dark pics it was just for a comparison) and as you can see here at 10mm: the whole thing fits in
It probably doesn't look like it, but I was almost on top of it to get that shot.
Now let's cut straight to it in 35mm focal length:
If it's too dark to see, just look at the blinds.