I'm using this lens on a 20D and in auto focus mode it has performed well. As I become more familiar with this lens I am very pleased with the operation and the results.
However, I have noticed a slight back focus problem recently. Originaly, I thought it was my problem getting used to the features of the lens.
In manual, if zoomed in all the way to get a focus and then zoomed out to frame a shot the focus does not track. While it is annoying to pay this much for a lens and have this problem out of the box, I recognize that it is repairable.
What is most disturbing is that when I called Canon regarding this problem I was told that this is a non-par focal lens and is designed to operate this way. I was told that there was nothing that Canon could do to repair the problem.
Franlky, I thought the answer was B.S. and called a local camera store about it. I was told by them that this is a common problem with this lens. They have sent in quite a few for this issue and when returned the problem is solved. I'll give an update when it comes back.
Clearly this Canon rep was not familiar with the product and I recommend that should you call Canon for answers keep in mind that their people are reading promps from a screen. If it souds like B.S. it probably is.
This situation won't change my mind about Canon products as I have been very happy with them for many years.