My main gripe about this lens is the lack of precision in focusing. It's almost as if the cogs making up the gears are too big, or that it operates using a stepper motor with not enough steps. I had to get three lenses before I found one that focused reasonably accurately. All three did the thing where it refocuses when already in focus if you depress the focus button. Due to a focusing issue, I sent it in to Canon for calibration (along with my other lens and Canon 10D camera). Unfortunately, the first time I sent it in, they didn't really do anything, so I had to send it in again. And finally a third time. The good news is that after the third time, it stopped doing its refocus thing. Another manifestation of this "refocus thing" is if you put the camera in AI Servo mode, mount the camera on a tripod and focus on a flat subject, say a meter or two away, the lens refocuses so wildly it almost seems to vibrate. It no longer did this after the third time into the shop.
I don't know whether the problem was my camera or lens or the combination, but this issue did not manifest with other lenses.