I've been using lensrentals.com for a year now, and have used it about 5 times I think, so I'll give you my semi-unbiased opinion on them.
One, their service is great. I actually just got a rental yesterday (a day early, thus one extra day with the lens(es) for free).... However, the TS-E 24mm I rented came messed up. The tilt knob was loose and fell off. I called them and they overnighted one to me, it's on the delivery truck as I type this. No problems there.
So far, from them, I've rented stuff that I can't yet or ever afford, or just wanted to try.
Last year, when I still had a crop body, I rented the 17-55/2.8 and the 70-200/4 for a week. Awesome lenses.
I rented the TS-E 24mm to try it out a month or two ago for a week's time, and it was quite fun.
For the big car show I'm going to this weekend, I rented the TS-E again, the awesome 70-200/2.8IS, and the 28/1.8 for dinner, indoor party situations.
I've rented a camera bag when I didn't have one, and a flash as well.
They take care of everything quite well, none of lenses I've used seemed to be ever abused. maybe some exterior scratches or whatnot.
I've even bought from them. They sell of their lenses and bodies after 2 years of service. They advertised a 17-40L for 630 in 9/10 condition. I bought it and haven't worried about it since.
Their service (so far) is bar none. The lenses come wrapped tight in foam, bubble wrap etc. They include a return shipping label, etc... It's painless, except the hit on your wallet. But then again, if you rent 2000+ dollar lenses, of course it's not going to be cheap.
All in all, I would not hesitate to buy or rent from these guys ever.