My experience couldn't have been more trouble free.
Booked the reservation online through a third party site. Arrived at the airport and took the free shuttle to the rental center which took 5 minutes and smelled like a movie theater! (Love popcorn!)
I went to the kiosk and punched in my Alamo confirmation number. The price was a bit higher so I was about to go to the counter when a nice lady came up and pointed out that I was early. She adjust the return time to compensate and the price was corrected. Confirmed a few screens and I was off to the garage.
A couple guys greeted and directed me to the row where I was able to choose my car. I booked an "intermediate" car so the cars available were: Mazda 3, Toyota Corolla, Dodge Caliber, Nissan Sentra (I think). A couple of the cars smelled kind of funky so I chose the Mazda 3, which looked and smelled the cleanest (it had 1500 miles).
When I returned it, I pulled in and a guy scanned the barcode on the car and gave me a receipt. Price was as before and I was on my way!