I just wanted to add my thoughts about this lens after about six months of using it all over the world. As far as the optics go, it pretty much crushes my old Canon 35 1.4. Autofocus accuracy is pretty good, and the general construction of the lens is almost "L" quality. That said, I have found one major problem with it. While in Bangkok covering the protests last week, the autofocus driver died numerous times. It was clearly connected to the humidity (which never got THAT bad, about 80% while I was there). I have been in contact with Sigma, and will update when I receive a resolution. Obviously, the Sigma isn't weather sealed, but if the lens can't handle a mildly hot and humid afternoon, I'm not sure if I can give it my trust anymore.
All of that said, unless you are working in weather extremes, buy it, it's a hell of a lens.