I'm not really sure why the rating on this place is so low--we had a really great experience. Granted, we had zero waiting and the attendant got us in and out quickly.
We chose a moderate-sized vehicle and they told us to pick one out that we liked best, which we did. All in all, it was a great experience. The only reason for the four-stars is that our attendant wasn't very personable or overly polite, but that's being really nit-pickety.
One thing to note about ALL rental calls for Phoenix Sky Harbor International is that they are located off-site in one main area. It's a really quick shuttle ride (3 minutes?) and they really keep you moving both from the airport and back again. We LOVED the shuttle drivers!