It's always a good idea to check the reviews on this site. My two cents is that the Sigma is every bit as good (maybe better) than the Canon. I have used both and own the Sigma. I am, like you, one who normally stays with the Canon brand lenses because they have been so good to me in the past. I don't think I would buy a Sigma tele lens. That said, the Sigma proved to be a few things to me when I compared them side by side. First, it's cheaper. Second, it's sharper (albeit barely.) In pixel-peeping images taken with both lenses, I believed the Sigma to be better. Third, the image is more rectilinear. In fact, it's more rectilinear by far than my Canon 28-135. Comparing architectural images taken with both lenses, the Sigma did better at the same focal lengths. Fourth, the build quality is just as good as the Canon (again, maybe better.) It being an EX series lens, the finish is very good, it comes with a lens hood, and the HSM focus is just as fast. Fifth, it works on any EF mount. It is usable on my Elan 7. Of course, there is significant fall-off and vignetting at 10mm, but I can use it at about 13-14mm without any trouble at all. Depending on your stock of Canon bodies, this may or may not be a selling point for you.

Hope this helps. I (stupidly) didn't save any of the comparison images, but could post some 100% crops of images taken with the Sigma if you would like.