I have had the best luck with the Canon service center at the Canon regional headquarters in Itasca, IL.
Josh Stoner, broadcast lens division
Canon USA
100 Park Blvd.
Itasca, IL 60143
ph: 630/250-6234
There are yahoos out there that are in this business and I've dealt with them. For me, it is worth the money to have the manufacturer work on my glass (and they provide a loaner if one is available!!!)
--Mike