Despite my loathing of the car I ended up with, Alamo in Phoenix deserves four stars for great service and great prices.
I made my rental car reservation via Southwest and got 30% off plus points, which actually made renting a car for a day cheaper than the shuttle! Picking up my car was easy-- once I got to the facility I was the first person in line at Alamo, and Dave did a great job getting me ready to go and saving me a little bit more money. It turns out I am very much not a fan of the Toyota Yaris and its lack of both power and an armrest, but that's not Alamo's fault, and the car ran just fine. Returning the car was equally easy; I think I was on the shuttle and on my way to the airport in under five minutes.
Next time I'm in Phoenix, I'll probably pay a few extra bucks to get a compact car instead of economy, but I'll definitely give Alamo another visit.