I shoot with a Canon 10D and most of the time use the 17-40mm f/4L for my wide angle shots. With the 1.6x conversion the 17mm is closer to a 28mm lens which really isn't wide angle. Since I could not use the Canon 10-22mm EFS lens, I was excited about Sigma 10-20mm lens. I just recently bought a Peleng 8mm fisheye and enjoy taking wide angle photos.
After a few photo outings with the Sigma lens I noticed a few things, please note, I am comparing these with my previous lens, 17-40mm f/4L. The autofocus felt a tad slower on the Sigma, I tested this in a bright room and outside in lower lighting at dusk. While I didn't compare the two lenses between each other at the same time, it was my personal feeling that the Sigma took longer to focus lock. I also suspect this is from the one stop slower at zoom lens apature. My previous lens is f/4 from wide to zoom.
Another item I noticed was the slight soft focus of the images when using this lens. Again this could be a softer copy than the others have posted but the images seemed to require more sharpening and didn't have the over all sharpness that the 17-40mm had. My 10D is also set to netural for in camera processing, and I do all of my sharpening from Photoshop. In the images I tested were from a varity of lighting and subjects, the ones that produced the sharpest where in direct bright daylight. Also the shots were using varing apatures, from wide open to f/14.
Overall I would still highly recommend this lens for any one looking for a ultra wide angle lens. The only other lens I know in this area is the Canon 10-22mm EFS which is not useable if you are using an older camera such as my 10D. The slight faults of the lens that I found are just my nitpicking, and it's still produces great pictures.
Oh, for any film or full frame shooter, this lens on my film Canon will work at 12mm. you can also use it at 10mm but have a serious vinetting at the corners (very similar to using a 8mm on a 1.6x body).