So far my experience with Canon Customer Service has been excellent (Sigma should pay attention).
I received an email yesterday with the information that my lens has been received and work has begun. They gave my case a repair number where I can check the status online.
I called today expecting to get an automated system to see if anything unusual was found with the lens but was transferred to a customer care representative instead. I gave them all my information and they told me that the repairs have already been made and the lens was in the quality assurance department getting the work done checked. I checked the online status a short time after that and they said it's already completed and will be shipped soon.
The rep told me that there was adjustments made to the mechanics and the electronics in the lens to bring them back into factory spec.
To wrap up, I sent the lens UPS ground on Tuesday, April 21st. It arrived in Irvine, CA on Thursday the 23rd and was accepted by the service department. Today, April 24th the repairs are already made and the lens is being prepared to be shipped back. Amazing.
When I had sharpness issues with my Sigma 10-20, I had to send it in twice. My lens would disappear and magically show up on my door step three weeks earlier. The only time I ever got any detailed information was when I called and begged them for an update. I received an email a few days later telling me the lens had been shipped. They have no online monitoring and when you call, all you're told is that it's in for repairs. I will never purchase Sigma lenses again.