iGary, I've used both more than once. I owned the Canon 10-22 for six months before switching to the 17-40 (because I started shooting film), and it was great. Very, very sharp. Minimal distortion and no vignetting that I can recall.
In the time that I had the 17-40 and was still shooting with a cropped sensor (before I got my 5D) I borrowed a friend's Sigma 10-20 for a couple of shoots that I need superwide. Having used the Canon in the past, I immediately noticed drawbacks and felt more justified in my spending the extra dough. First off, it was slower to focus, and had a real hard time in low light. The build quality did not feel anywhere near as solid as the Canon. Then back at my computer, the IQ shortcomings were evident. Overall not as sharp as the Canon, and drastically softer in the corners and edges. There was also noticeable vignetting.
So there you have it. To me, there is a clear advantage to buying the Canon 10-22. And I am no biased fanboy, I own the Sigma 70-200 f/2.8 and love it. In this case, however, I side with Canon.