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.
Seems there ought to be a DIY way of adding an o-ring at the camera mount.
If the Canon EF 35mm f/1.4L is weather sealed, I see no evidence of it. I suppose it must be, though.
I'm curious if any of you have done astrophotography with this Sigma lens. If so, is there much coma toward the borders, and if so, how far does it need to be closed down before that goes away? I'm sure somebody has talked about this before, or perhaps over at lensrentals when they imatested it...but I forget.