I had a Canon 50 1.4 and was very impressed. Then I read the reviews of the Sigma, so I had to try it out. paid about $500 in Switzerland, about $80 more than the Canon. Considering this lens' primary purpose for me is shooting low light wide open, it excels in every way vs the Canon. Focus is faster, missed focus about the same frequency, DoF is razor thin (which has its own pros and cons I suppose), and it feels much more solidly built. I don't mind the heft/weight, in fact I like it.
Overall definitely worth the difference in price. Sigma did a good job here. For its purpose, a solid 9/10. Canon needs to follow.