I just got this lens and spend some time shooting over the weekend, and I'm seriously impressed. In term of sharpness wide open, I feel the 135 f2 is still better wide open in terms of contrast & sharpness. But what it lost to 135 f2, it make up with the amazing dreamy effect of f1.2 at 85mm focal length, even on my 20D.
The weight was causing some pain at my wrist just after 40-50 shots, since I don't have a vertical grip for my 20D. But I soon got use to it. The auto focus was quite decent, considering all the negative comment on it. It does become challenging trying to keep the shallow DOF focus on my active toddlers.
I don't really think this is a low light lens, since it will be challenging to focus at low light especially for moving subject, but it really shine when there are decent light available, I can leave the focus on AI-Servo on 20D, and the lens can still track reasonably well.
I am keen to try it on 5D/1Ds if I can eventually get a good price for the body.