+1 on borrowlenses.com. I've rented from them a bunch and I've got nothing but good things to say about them. I pick up my rentals in person from their main office and everyone that I've dealt with there has been pretty cool.
What you renting? I love my 500 f/4 with and without a 1.4X. It is the bomb with a Gimbel head and a large tripod. The 500 is actually lighter than a 400 2.8.