Okay, you asked for it.
Sent it in last fall to have them look at a focuing issue. Called them a week after sent to confirm they got the lens, and to find out what timeframe I'd be looking at. The person I was transfered to seemed not very professional, he did tell me it arrived unharmed, that they'd contact me within a week to let me know if they found any problems, and also added "how busy they are".
I let two weeks pass before calling for an update. They couldn't put me through to anyone, so I left a voice message. Two days later, no reply. Called again, no one to take my call, left another voice message. No reply for another day. After a two more days of this, I was insisting on talking to a real person, they said there wasn't anyone to take my call (was always the same person answering the phone, they must have a very tiny staff), left another voice message. This went on for nearly two weeks, never actually talking to anyone but the receptionist. Even tried calling the head of customer support in their HQ and didn't get a reply.
Finally got feed up, sent the President of Canon Canada a fax stating the the problem I was having reaching any real person at their repair facility in Calgary. Less than a half hour after sending the fact, I got an email from a Customer Support person at the Calgary facility saying they would "fast track my lens"

and I'd have it back shortly.
Another week passed, no lens. Then I get another email saying they'd lost the lens, and would send me a cheque. The cheque wasn't for actual replacement costs, but for the sale price I bought it at (about a $100 difference). This lens was ten months old, and under warranty. So, for a month, they actively avoided taking my calls and telling me they'd lost the lens. My guess is, they were looking for it and hoping they'd find it before dealing with me. Pretty awful experience and I never want to deal with them again.
Just thought I'd add that before I sent it in, I read another horror story (in a photo forum) of some guy who sent his camera into a Canon repair depot in Chicago. He apparently received the office garbage (banana peels, waste paper, sandwich bits, etc) back with his unrepaired camera. I took it as an isolate, unsupported story and didn't worry...but now, I fully believe it.