I've had this lens for a couple of years, but am really struggling to get sharp pictures now. I've tried it on a 400D and a 7D and both show the same results.
I had the problem with it getting stuck at the end of the zoom range and so had to take it apart to repair it, but it hasn't been the same since. (Note that this repair leaves all the optics intact, it just involves the assembly around it)
It isn't even like the focus is out a little, some shots have nothing showing as sharp, whereas others can be fairly alright.
I've tried the lens focus adjustment on the 7D but it seems fine on a tripod, just not in real life.