We used Art's Shuttle with wonderful results. Small van so when the driver finds you at luggage, he whisks you away to a van that is all yours - no other passengers. I've also done Mears, but have to wait until those big old buses fill up.
Art's also picks you up at the pier when you return, helps with your bag to the van and again, you're off with no wait.
Carnival Elation March 11
Carnival Imagination Sept 07
Carniival Sensation Dec 06
RCI Sovereign of the Seas Sept 06
Carnival Miracle Sept 05
Carnival Glory Sept 04
Carnival Fantasy Jan 04