My only non-Canon lens is the Sigma 10-20mm f4-5.6 EX.
When I bought it (over 4 years ago), it was a toss up between that and the Canon 10-22mm. Reviews indicated very similar sharpness, (in general Sigma slightly sharper in centre, slightly less sharp at edges - but that is noticable only while pixel peeping), moderate levels of CA and decent handling of flare.
The Sigma's build quality is higher (more sturdy, smoother zoom & focus rings) - whereas the Canon focuses a bit quicker (not usually important for ultrawide zoom lenses, and particularly not for how I use them). So I tested the Sigma lens and was very happy with the image quality - and there was only a very minor difference in colour cast between the Sigma and Canon.
My Sigma 10-20mm is particularly sharp at 10mm, corner to corner. It has a slight focus and decentring issue at some focal lengths / focus combinations - again only noticable when pixel peeping. I can overcome both these issues by manual focus (I keep it on manual focus). With manual focus for some reason it seems to over-expose about 1/3 EV, but again, I just turn my 7D's rear wheel to compensate, so basically all is good.
The main reason I went with the Sigma was that it was about $400 cheaper than the Canon! Also, it came bundled with the lens hood. The Canon lens hood needs to be purchased separately, and it is annoyingly huge. I bought a cheap ($5) 'pinch / snap' lens cap to use when the lens hood is on (I always have the lens hood on).
I'm very happy with the Sigma 10-20mm EX. I use it often. It's very portable - and produces high quality ultra wide images. I use it a lot for landscapes, sunsets and sometimes architectural shots (indoor and outdoors).