I don't really like mine, seems like the focus is off. I can put it on a tripod and focus on something.....then focus again, and watch the lens move like it's trying to re-focus. After about 3 times I can push the shutter botton and the lens will not move and stay locked on the subject. None of the other 2 L lenses I have do this. And this is the second lens, I took the first one back because of the same issue. Also this was the case on my 10 & 20D. I shot some motocross photo's with it and they came out blury as hell. I need to do some more test with it, but I can't be bothered......I just seem to avoid it. I did test it on a stuffed animal where I focused on the eye and the eye was not sharp, however right under the eye was. I think I am going to have to send it to Canon and have them check it out.
For those of you that have this lens I would be interested to know if your does this jumping back and forth thing when you focus on the same point.