I had been trying to choose between the Canon 50 f/1.4 and this lens for some time (as many are, I'm sure), but after hearing so many stories about sketchy AF accuracy on the Sigma, I decided to hold off.
After hearing that a fellow FM'er had a sharp copy that he had returned to a local store (to fund the purchase of a 50 f/1.2L), I ran right over to the store to pick it up.
Well, let me tell you this lens is fantastic. It has lived on my 1DmkII for the last few weeks. AF accuracy is terrific; no front/back-focus issues at all. AF performance is a bit slower than many L primes, but is certainly as good or better than the 50 f/1.4 and MUCH better than the 50 f/1.8.
The reason people buy this lens is the bokeh, and I can confirm that the bokeh on my copy is stellar; to date, the best bokeh I've seen is the 85 f/1.2L, but this one is a close second. I know it's not to everyone's taste, but if you like buttery/smeary bokeh, you'll love this lens.
Build quality is much better than the Canon 50 f/1.4. The Sigma is fairly sharp at f/1.4 and very sharp at f/2.0, which is where it lives most of the time on my camera.
If you can find a copy with no AF issues, this lens is definitely worth the premium over the Canon.