Sure are the way to go. We have said this before in this forum. The "set price" is not exactly that. There is a port entrance fee the taxis/limos get charged and their could be a long wait in line to drop off which the taxi will charge you for. BUT, if you have more than 1 person in your party, the taxi is cheaper AND you don't have to wait on the bus.
You can get the airport taxi attendent to get you a large taxi (van or suburban) if you have a lot of people/luggage. We fit five (3 adults 2 kiddos) in a Suburban taxi without too much trouble.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007