I tried two copies and on both autofocus was accurate and pretty fast (5D II / 7D) - nothing to complain about.
Optical the lens is perfect to me, never saw such a great performance at 1,4.
The first copy had a squeaking HSM so I returned it. The second one ist much better in terms of squeaking :-))
I heard that HSM sometimes can be "squeaky".
Some samples if you like: http://www.flickr.com/photos/62909363@N02/sets/72157632986987422/
My sigma 35 doesnt squeak but i have a 17-40 that sounds like there's a small mouse inside operating the AF. Works perfectly otherwise though.