I have used my 50mm1.2 on a 400d body, and thought that i was experiencing some of the focus issues i had read that the 50mm had, but when i put it on the 5d it was a totally different story; it worked beautifully. basically my problem was that it wouldn't be as sharp as i would have like or i would think i 'had' a certain shot only to discover the focus was way out.

the comment regarding never wanting to use your old body is true though. you wont go back.