I dunno if I would say avoid but definitely make sure you get a nice copy. Sigma is known for having some QC issues and the 50 1.4 isn't immune to it. There are many many reviews on how good the lens is. The build quality and sharpness are all way better than the canon version, not to mention the beautiful bokeh on the sigma. So if you are getting a sigma , try buy it local to make sure you are happy with the copy.
Definitely avoid it. Why support a company that puts out a defective product? The Canon 50mm f/1.4 is just as good without the headaches.