As a Emerald member, its extremely convenient to rent car from National. Exit the terminal via underground pathway and you come right into National lot. The Aisle is usually stocked well and staff is always around to help out.
Sometimes the selection is not that great, especially if you arrive late into the night. There was one time when I landed late into the night (12:30am) and found the lot closed. had to go over to Avis to rent a car that night.