Sigma 20 1.8 vs Canon 28 1.8 ??
I've owned Sigma 24mm f1.8 and optically that lens is great. However, AF is awful. I've tried 20D, 40D, and 5D and on all bodies it focused properly at close (macro) range but during a wedding shoot it would miss the focus all the time. After a few weddings I've stopped using it because I couldn't rely on it at all. On static objects (shooting wedding rings, shoes, etc.) it was great.
Even more, on 5D when using AF assist beam the lens would SEVERELY front focus and if I've turned the flash off it would focus better.
After a while I've realized that I can't use such lens and I've got rid of it. It is unfortunate, because I really liked the build and the optics.