I've probably rented cars 20 or so times on vacation, and all I can say is that I've never had a good experience with Alamo. I've used them around 5 times on those vacations, simply because they were either the cheapest or they came as part of a package from a travel agent.
The last time we used them, in 2001, was the last straw. We were given a child safety seat that looked like a reject from Goodwill and was totally unsafe. We we left the parking lot, I realized that the brake pads on the car were worn down to metal against metal. I went back to the counter, and the ONE customer service person on duty (it was the week after 9/11) couldn't care less about either situation. When we returned the car, they nailed us for an entire day's extra rental at rack rate because we were 1/2 hour over the 24 hour period.
On our last trip in 2005, I gave National Car Rental a shot, and it was the best experience I ever had with a car rental company. I did the entire transaction on line and walked DIRECTLY TO THE CAR at the airport. No paperwork, no waiting. The car had exactly 36 miles on it. Returning it was just as easy. No surprise charges like with Alamo.
The ironic part is that Alamo and National are owned by the SAME COMPANY!
If you use the discount codes on Mousesavers.com, the rates are competitive, (if not better) than all the other companies. I highly recommend them.