I've rented from lenrentals at least a dozen times. First, if I'm not sure if I want to own a particular lens, I'll rent it for a week. For example, I rented a 15-85 and 24-105 together. (bought the former. IQ was great, the dreaded CA I read about was not apparent)
Second, there are some lenses that I only need once or twice a year and I can't afford to own.
Finally, I love the fact that they'll ship it to a UPS store anywhere in the US. I travel by rail a lot and love the convenience of having a big heavy lens waiting for me at my destination.
As the previous posters have said, the customer service is AAA+. Never had a shipment come late, never had a lens come that was in less than pristine condition. If you have a comment or question about a rental, someone gets back to you ASAP. I think they do a great service for photographers especially 'prosumers' who like to try different things but are on a budget.