Been shooting with this lovely thing and I have found it a stubborn lens to get right. I've used the 70-200 f/2.8, 24-70 f/2.8, 400 f/2.8 all with little focusing problems but this 85mm sucker at f/1.2 is really hard to get right.
I use a 1D2 set on AI Servo with center single point AF focus. I've found that my lens is sharp and the amount of backfocus is not a concern during tripod tests.
The problem I have is keeping the focal plane in focus between the time the camera focuses and when I press the shutter, there is a slight delay even in AI Servo and the camera shake or head/body movement will ruin the shot. Sometimes the picture frontfocuses, sometimes it backfocuses and occassionally I hit it right on.
Any recommendations on what you use to shoot? I was just thinking of bursting several shots and hopefully one will be in focus.