I have bought two just recently. A 15mm 2.8 Fisheye. Wonderful lens, could tell a tiny bit that it was not brand new, but glass is perfect. After a few days I took it outside while it was hot and after a few pictures the lens stopped working and gave me an ERR01 message on a 5DII. This kept happening. Sent it back to Canon, they turned it around in two days and said they couldn't find any issues. Tried it again and it failed again. Sent it back, this time mentioning the high heat. They checked it again and found a problem under high temperatures and replaced the whole aperture assembly and I have it back after a few days again. Works perfectly now.
Bought a 24-70 2.8L. Lens looked brand new, except there was a huge amount of some type of dirt, like tiny fibres or something inside the lens. I just sent it back to have them clean it. Otherwise it worked just fine and is pretty sharp.
So all in all, I guess I haven't had that much luck. However, their technical service is very friendly and extremely fast. My lens wasn't gone for longer than 4 days each time. Yeah, I had to pay for shipping it back to them, so that does add to the bill. On the other hand, if you buy the lenses used from someone and a few days later it turns out it doesn't work under high heat, you are on your own. I would purchase from them again. However now I know to inspect them immediately and thoroughly under all sorts of conditions.