I have used the service center in California, to repair a gritty tension ring on my EF 100-400mm L series lens. (Turned out that some kind of bearing inside the tension ring had shattered, leaving a very rough, gritty feeling when twisting it.) The inter-personal service was pretty good, people were friendly and moved me through the process pretty quickly during the initial stages. I had some up-front questions that I wanted to ask before sending it off, and that was where my experience got a little strange.
I was eventually forwarded to a man who sounded like he was in his late 50's or early 60's, sitting in his dimly, oddly yellow-green lit garage, tinkering with lenses and rummaging through old metal cabinets. It was one of the most ad-hoc, down-to-earth, "out of the remote wastelands" kind of conversations I think I've ever had with a support center...I really felt like I was getting answers from and shipping my lens off to some little shack on an island out in the middle of the arctic circle, manned by one old geezer who Canon apparently thought was an absolute lens repair wizard.... VERY DISTINCTLY ODD EXPERIENCE.
That said, the service was pretty quick, I think it took just over two weeks (shipping time included), and when I received my repaired lens, it worked great. Still works great. There were a couple days of lag time from the point where my lens was marked as received at the service center to the time it was actually worked on. That spanned a weekend, so it felt like a long time, when I think it was only two or three days at most. The time actually in service was just a day, if I remember correctly, with an extra lag day after service before it shipped. One thing that was a bit frustrating was the fact that a service order number apparently could not be created until the service center actually received my lens and put the physical item into their pipeline. That made it difficult to track until shipment arrived, AND they had time to process it. I eventually found out that I was able to track by my serial number before an order number was created, but after the order number was created, I had to track by that and not the serial number (still getting the feel of an arctic wasteland shack here....)
If I ignore the odd conversations with the optics wizard in the arctic island shack, I felt the service was pretty good and decently quick. If I include the odd conversations...well, it can't be anything better or worse than simply odd. I would get some opinions on the NJ and VA service centers before sending your gear off to the CA one...just to see if you can get a more refined experience.