Honestly, I've never rented a lens. I haven't ever been in a situation where I've had a defined, short term need for a lens. I don't believe in renting a lens to 'try it out' as I feel that's money better put toward the purchase of a new lens. If there's a lens that, after thorough research, I'm still not positive I want, I wait for one to come up on Craigslist and buy it used. Since it's not a need
(else I'd buy it new), a can be patient. I pay no more than 70% of the current/recent new price. That way, if I decide I don't want to keep it, I can re-sell it generally be no worse off. I've done this with several lenses (200/2.8L II, 300/4L IS, 70-300 DO, 1.4xII, 2xII), and overall I've had a small net profit. I also bought the MP-E 65mm and 28-300L which I still have, although I may sell the superzoom. Basically, I get to try a lens for as long as I want, essentially free (or better!).
I have tried lensrentals.com and borrowlenses.com and was happy with both.
Good to know about borrowlenses - they have a pickup location near me, so no shipping charges if I ever do
need to rent something.