If you don't use a known company like Lee's, then please be careful. You will have some locals that will offer you a ride around Jamaica for a fee. They are trying to make $ anyway they can, which isnt always the safest way to go for tourists.
Just make sure the taxi cab company has some kind of legitimate sign on their car. I've heard so many great things about Lee's Travel Tours, that I dont think you could go wrong just using them :~)