I've been lucky enough to find a excellent copy second-hand for Eur. 300,-, after I thoroughly read a lot of reviews to be informed about all pros and cons of both the Tokina, Canon and Sigma UWA-lenses.
First field-tested this lens on a trip to Germany and Luxembourg, and I'm trilled with the results. 10mm is really fun to play with, and I no longer can imagine having an UWA starting at 12mm.
The sharpness and colors are great throughout the whole image. I've seen no softness or CA in the corners yet, as it is frequently reported here. Guess I'm lucky, after reading and hearing about Sigma's QC...
The HSM is silent and fast. Makes me want to buy more USM or HSM lenses, although the AF tends to hunt a bit in darker circumstances (but as far as I know, this is common and usual in darkness).
The lens is built like a tank. When compared to the kitlens I got when buying my 450D, the kitlens seems like a piece-of-crap toy. The Sigma's focus- and zoom-ring are well damped and the lens barely extends when zooming to 20mm.
On the downside:
Don't know who designed the lenscap, but it cannot be used when the hood is attached. Silly.
Apeture starts at F4, which makes this lens the slowest in the UWA-department. By the way, this didn't bother me at all while taking pictures in darker places, such at churches. At 10mm, F4 still is pretty good to work with.
Although flare is pretty common at ultra-wide angels, this lens sometimes shows an above-average amount of flare. Just don't try to make pictures of the sun, and it won't bother you ;-)
For the price I paid, this in an absolute winner and it is likely to become my "always-attached" lens.
For the price paid in regular shops, just make sure you have a copy that actually passed reliable QC in terms of sharpness and CA and you will most definitely be satisfied.