I used
www.lensrentals.com once (the only time I had rented any gear). I needed a Canon 40D and a 70-200 2.8 IS. The camera came 3 days early which was great. I was shooting something very important but the day before the shoot the cameras auto focus quit working. So, I called and there was no way they could get another camera to me in time they DID refund my rental feee for the camera.
I know that theres always that chance of gear failing so I wasnt overly upset (disappointed yes, but not upset) but I have to say that I was VERY impressed at how concerned the staff was that this had happened and that they rectified the problem as best they could.