I tried two copies, both had serious backfocus issue. I read all the reviews here, but I had to see it myself! First one I returned "no questions asked" immediately after quick test. I really wanted a working one and I ended up buying a second one from different dealer. He was really helpful and I was assured that if there would be any problems, he would help me. Backfocus again, two times calibration in Canon service with the body (5D) would not correct it. I still don't know what the problem is/was. I expected it to have focus shift when stopped down slightly, but it was a suprise that the two copies I tried had serious focus issues at the maximum aperture. I ended up changing it to 85L II although it is no my favourite focal length. No problems at all with that one.
I would say that this truly is a great lens, when focus is where it should be. For me it was impossible, because I like to shoot very close with 50mm and it just didn't work. I would definitely buy this one again if I would be sure it works. It would have to be 10 lenses in a row to try out. I would not recommend anyone buying one with backfocus and trusting 100% that Canon service will fix it. I guess I will have to wait for new version or try the 50/1.4.