Just to be clear - when you prearrange your transportation, it doesn't have to be with a shared shuttle service. In fact, most of the providers listed in Transportation link offer a private, chartered car or van. Therefore you are free to travel to Negril at your own pace, stopping anywhere you want to along the way.
We used a shared shuttle on our first trip to Negril, and will never do that again. We have hired Linston for our past few trips (most recently last week) and can definitely recommend him for airport transfers and excursions.