I have had the sigma 10-20mm for about five months and I love it. You get great wideangle images, about 110 degrees angle of view on my Nikon D40. That means if your standing in a corner of a room, you'll get the whole room plus a little bit more
It is a sharp lens except for the EXTREM borders at 10 mm, but this is no big issue at all. It could also have had a larger aperture since I shoot mostly indoors, but at that price, you just have to adore this piece of glass.
AF is fast and accurate.
If you plan to shoot pictures with flash, you have to bounce it. Otherwise you'll see the edges of your flash on the picures...