I´ve had the lens for a month now and was using it with a UV filter attached to prevent possible damage to the front element.
I bought the lens for the extra range it would give as an addition to the 70-200 I have.
Was about to return the lens as all pictures seemed out of focus. With the 50D results seem to become better with a backwards adjustment of +20. However, quality was not close to the 70-200 F4 IS I have. Before bringing it to the shop, I repeated testshots I made earlier for the micro adjustments, but now without the UV filter, and the lens now produced sharp images.
After further test shots (birds, buildings etc.) I am very happy with the lens. Lens is build like a tank, and the push pull system is very easy to get used to.