I recently picked this up used from KEH. I had heard great things about the Canon 15mm Fisheye, but didn't want to spend another $200 for the Canon. Reading a number of good reviews of the Sigma, I decided to give it a try. So far I have only taken a few dozen test shots using a Canon 1ds Mk i, but I have been very careful with those. Contrary to what some reviewers have stated, my copy at least is not "tack-sharp" at 2.8. I agree with what a couple other reviewers indicated: that sharpness at 2.8 is "usable" but not stellar. Of course, I'm studying 100% crops. That's really the only way to make a true comparison among shots where sharpness is concerned. Things improve at 3.5 and 4.0. By 5.6 I would say the street sign I was shooting looked "tack-sharp" at 100%. So at 5.6 and smaller apertures, I would say center sharpness is very good. Corner sharpness is good to very good.
In comparison, for example, to my Canon 400mm f5.6 L, the Sigma is definitely not as sharp even at f8. Where it really suffers, though, in comparison to that lens is in color rendition. Next to the L lens, the colors look rather "flat." They're not awful, just nowhere near L quality. I have never tried the Canon 15mm Fisheye, so I don't know if it renders color better or not. I used to own the Sigma 10-20mm wide angle and sold it because colors lacked snap also with that lens.
I will keep this lens for a while at least to see what sorts of uses I can put it to.