I have a canon EF85 F/1.8 which is a little back focusing (tried on my 300D and a EOS50 film body). I bought it second hand on CS with 6 month shop warranty left, so I sent it in for repair. Two weeks later, I got it back with the statement that the lens is OK... And they let me try a new 85, which showed the same problem.
Then they suggested I send it in to Canon Service, and they'll cover the charge for me
So it went to Canon. The technician there told me it is back a bit but still considered within specs. Then it was adjusted, I paid 63 dollars for the service - and brought the receipt to The Camera Workshop And they issued a cheque right away!
Man that was some GOOD experience with them.