I've used AVIS for many years through work and have cottoned on to booking my long haul holiday (US) flights with a car, not only saving money, but you only pay a deposit .
Amongst some of the other benefits listed here already, when you arrive at the destination (provided you gave them your customer #), you go straight to your car and straight out the car rental centre.
I've seen humongous queues at some US airports, especially in peak periods.
Works 100% for me every time.