One of the rare activities off of the general tourist path in Jamaica, Roaring Rivers Park is an underwhelming attraction if you're expecting whitewater rapids or epic cascades. If you're looking, instead, for some moderate waterfalls, a few swimming holes, and limestone cave located on a former private estate then your adjusted expectations will be met.
Located about an hour's drive southeast from Negril along the A2 (about half an hour past the Blue Hole mineral spring), we suggest booking a tour ahead of time and paying the $15 USD or so per person (plus a tip for the guide). This will undoubtedly be less stressful than negotiating a tour with one of the "guides" that you'll meet at the entrance to the grounds. If you do choose to pay one of these gentlemen, be sure to negotiate all aspects of your tour (tour cost for your whole party, tip, what the tour will consist of, etc.) prior to seeing the sights, otherwise you may find yourself strong-armed into paying much more than you intended.
That being said, there is still much to enjoy at Roaring Rivers. The river swimming with the locals is refreshing and the cave, if lighting is adequately provided, is quite large and interesting. Just remember that this is off-resort Jamaica, where poverty drives people to give unofficial tours of public attractions or to sell homemade wares in the parking lot. If this is a concern, enjoy Roaring Rivers as a tour bus stop; we won't blame you.