I'd like to share my bad experience and eventually ask for what to do with Canon at this point.
Last year I bought a 70-200 f4 L IS lens for my 400D, one of most appreciated lens and of professional level.
After some tests, I discovered that focus was not consistent, also with frequent cases of apparent motion blur even with very short shutter time (like 1/1600 sec @ 200 mm, equivalent to 320 mm).
I sent the lens to Canon under warranty together with my camera since the problem could be in both, though with my other lens I don't have such problems, in particular my 17-55 f 2.8 is always tack sharp.
After a couple of weeks I got back my camera "calibrated" for 40 euros (at least they say they did it) while the lens was said to be ok.
New tests and still focus problems.
I sent back the lens and they replaced the focus motor and the IS mechanism. Tested again and still problems.
Canon kindly lent me a 40D for a weekend to do some tests with another body. Some better results with respect to my 400D but not OK as it should be with such a lens and a camera. Sent back the lens.
Canon replaced my sample with another one at no cost telling me please not to ask for more. New tests (also with another body) and my mood becoming very frustrated. Again problems and today I made a test that confirmed that I am not a mad guy and there is a problem:
camera on a good tripod and with remote shooting (and no earthquake in the period...)
very good light conditions (outside in a sunny day)
targeting a still subject (a parked car) @ 200 mm
focusing repeatly, the focus ring MOVES slightly from one shot to the other (about 1 mm that is significant in a tele), giving good and bad shots.
See as an example the attached comparison between two consecutive shots.
Now the question is: what to do now? Is my camera faulty just for this kind of lens or is (again) a bad lens sample??
The warranty is of course over now and I spent 1000 euros for an L lens that is not an L lens.
I am losing hope that Canon can deliver good quality equipments (already 2 years ago my previous 17-55 was severly inconsistent with focusing and after 6 months of back-and-forth, Canon admitted there was a problem and they replaced with my current sample that is very well focusing)....
Any suggestion is very appreciated. Thanks a lot.