I've had a lot of problems with the ParkNFly shuttles for other than the valet parking. They've often been infrequent and crowded. Once, the driver fell asleep on the Economy run. Don't expect much and hope to be lucky but at peak times it's a challenge. Direct Flight is the cheapest but they used vans that had serious mechanical problems and I gave up on them. Skyway and other small operators can be okay, especially if they remember to clean the van. The outdoor GTAA lot is convenient to the train and worth considering.