The problem with transfers is that you are at the mercy of the bus schedule. You can arrive and be ready to go to the ship, but end up waiting around for another plane to arrive to fill up the bus.
As long as the taxi ride is not too far, I take a taxi (and Cruise Fanatic already answered for San Diego) The cost works out better when you can all fit in a big taxi (like a van or SUV taxi) then to pay per person for the transfers.
Miami is in this category. Taxi is probably better. And if you look further down in this forum, you'll see that I found that renting a car in Orlando and droppping it off in Port Canaveral was MUCH cheaper than using the tansfers.. So, its wise to investigate the costs before purchasing the transfers, you can alway call your planner back an add them if you find you need them.
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