I've used the econmy lot a PHL. It's just passed the F terminal. It was very convenient getting to the terminal. We just parked the car and only waited 3 or 4 min. for the shuttle. There are little covered stops throughout the airport. Returning was a whole different story. We arrived in Terminal D, which is the fourthstop for the shuttle, and it was about 11:30 PM in the summer. Shuttles kept passing us up and we waited 45 min. The shuttles filled up at Terminals A &B. The shuttle stop is right outside each terminal.
We've never used Avistar? but we have used Pacifico. They are terrific. You go to their lot and one of their drivers drive you in your car to the departure terminal. They then drive your car back to the lot. When you arrive, you dall as you're waiting for you luggage, they pick you up in your own car and put all the luggage in for you. There is a AAA discount and it doesn't cost more than the economy lot.