I have the Sigma and it is one of my favorite lenses. If this 35 drops to the level the 28mm IS was a few days ago ($550@Adorama), I may just pick one up for the same reasons. I almost grabbed the 28 IS as a replacement for my EF28 f1.8, but 2.8 isn't quite fast enough. Unlike some around here, I would like to have IS on ALL of my lenses...
I recently rented the Sigma and bought the new Canon, used them mostly outdoors at night in very low light, and am very impressed by the image quality generated by both of them.
There is, however, one area where the Sigma plainly trumps the Canon, and that is coma. In the photos I took it was negligible on the Sigma wide open and completely gone by c. f2; and where it *was* visible, it didn't extend all that far into the image. I didn't try either lens on a crop sensor camera, but I wouldn't be surprised if it didn't show up at all when paired with such a camera. On the Canon it's not only much more noticeable, but extends farther into the image (though it's not as bad as the 28mm 1.8 or Nikon's 50mm 1.8D, where it covers all but a tiny patch in the middle) and would likely affect images created by a crop sensor. I suppose I should note that it's a rather superior sort of coma: the "sails" or "wings" (or whatever you call them) are crisply defined, not amorphous smears.
In any event, if you find coma as annoying as I do and take photos in situations likely to generate it, you'll want the Sigma, which not only trumps the Canon IS in that regard but (as far as I can tell) just about any other fast, wide prime. (I won't be keeping the Canon.)