Took some weeks to research the UWA options for use with a Canon 20D. Since I have a 17-40L, I wanted to go as wide as possible, which ruled out the Tamron and Tokina. Played with in-law's Canon EF-S, and thought it was good, but too expensive--it would have cost more than the 17-40L, and you would still need to purchase the hood for the EF-S!
First copy of the lens was bad (SN starts with 102)--it would focus hunt even in bright conditions with center focus. The hood was slightly tight fitting. The images were fine in the middle, but one of the sides was always quite soft, and has worse CA. All indications were that it was a de-centered lens. I returned it for another.
The second lens I received (SN starts with 103) is excellent. The hood now goes on smoothly, and the focus speed is fast and silent; no focus hunting! Sharpness is very good to excellent. Colors are very slightly Sigma-warm compared to the typical Canon-cool, but I shoot RAW mostly, so not a big deal to correct in post if desired.
CA is minimal to non-existent. Distortion at 10mm is okay and adjustable with PTLens, but admittedly, my in-law's EF-S shows less distortion. Vignetting is noticable wide open, but again easily corrected when necessary in post. It also seems to handle flare well.
The build is very good--I like the EX finish and I think it feels better than the Canon EF-S equivalent. Zoom and focus rings are smooth and well-damped. The included front lens cap is useless when the hood is on (I replaced it with a Tamron center-pinch). The rear cap is blah, although no less effective than the Canon equivalent.
Overall, it is a fantastic value compared to the Canon EF-S. Best of all, I'm having fun again with the wide FOV! UWA's are not for everyone--the wide DOF, distortion and FOV pose quite a compositional challenge for most amateurs. But a little bit of patience and experimentation will go a long way. And thanks to Sigma, we can spend less and get equivalent quality to an OEM lens.
Unhesitatingly recommended, provided you get it from a place that will allow exchanges due to QC.