I'd go and just get a taxi, less hassle and quicker. Its also less expensive than the transfers.
Just go to the taxi stand as soon as you get your luggage and they will write out a ticket for you, then go out and get in the taxi line, very quick. They will adjust the cost depending on how many bags you have and then just add a tip, should be around $20.
Go with Donna's suggestion. Some other folks who took the transfer said it took longer because they had to wait for other passengers on arriving flights.
Probably takes about 20 minutes or so, its all highway driving. Hold onto the edge of your seat! They are CRAZY down there for driving! Two of my co-workers who are from PR warned me about the driving down there & they weren't kidding. MA & NYC drivers have nothing on them.