^People's experiences will definitely vary. My local store doesn't even carry Sigma because of the said quality issues. I've been one of the more lucky ones (I guess?) and haven't had any issues with my Sigma. The AF isn't as speedy as Canon's USM in the L series, but I learned to work around them. I've owned and own lenses that are considered best in class and I rank the Sigma very high up there. That's too bad that the quality varies so much - because if you get a good copy, it is VERY good.
Yea, I really don't know what to say about why some people have more difficultly than others with Sigma. I was never disappointed in any other aspect of the 50mm, just it's AF reliability. For a $500 lens, it's optically much better than anything in it's price range.