Smooth car rental and easy check-in and out... the shuttle service that they use is shared among all the car rental agencies and they take you to and from the different PHX terminals quite efficiently.
The car we got was a bit dirtier than I expected (backseats were stained and we found a used cup under one of the seats), but it worked fine. My major complaint is not a Budget issue, but more of a Phoenix airport tax issue, where the PHX airport locations have a ridiculous mark-up of about 30% which is steep compared to other airport locations.