Apparently some copies of the lens have issues with consistent af and so perhaps I've been lucky there (after considerable micro adjustment to the af) but there are great copies of this lens about and buying from a reputable retailer always allows you to swap your lens for another if you do indeed get a dud. Amazon are great in this regard.
I own the Sigma f/1.4. Took me two copies, but I got one with decent AF. Being that it's still a 1.4, AF can be hard to nail, but when you do the results are outstanding. I had the Canon 1.4 and sold it for the Sigma. Real happy I did.
Here's a recent shot with the Sigma: