This last weekend I rented a Canon 135mm f/2 prime lens for taking shots of the kids and so on. I've rented lights before, but what surprised me was the really low cost of renting lenses. I don't know if this is common in other cities, but the cost of such a lens is over 1000 Can$, but it rents for a weekend for $25 (Friday afternoon pickup, return Monday before 11). In this case you can also apply the rental to the purchase price, so it is really an extended try before you buy idea. At that price, I could really justify it for shooting stock photos - doesn't take too many sales to pay back $25.
I could see this being great if you want to rent a specific lens for the occasional wedding or family photo shoot, or to get some specific shots (Tilt shift or fisheye?) without the expense of the full price.
On the one hand you may think that you would break even after renting 40 times, but it is more complicated than that. After 40 rentals you have no lens. If you were to buy it for $1000 and use it for three years, you would still be able to sell it for $600-800 (professional lenses seem to hold value really well). So really the payback is closer to 10 rentals.
It was fun fooling around with an f/2 lens for a while, the shallow depth of field is great to work with. This particular lens is known for being very sharp wide open, and it didn't disappoint me. That said, I've been thinking of getting a prime lens for a while now, and found it was quite tight on a crop sensor camera. It would be limited to outdoor shots, pretty much - but that is the case with my telephoto zoom as well.
Next time you want to purchase a lens, consider renting it first - you may be surprised at how different the lens is from the reviews you read. This barn shot was with this lens - I'm sure some of my other shots with blurred backgrounds will be appearing soon...