These are just my thoughts on whether or not to rent a car when you are on vacation here in Puerto Rico. The first thing to consider is how long will you be on vacation in Puerto Rico and do you want to see more than the beach and Old San Juan.
If your vacation is for several days or you want to take a quick trip outside of San Juan then I would suggest you think about car rental and here are my reasons:
1. With car rental you can arrange your day trip to your schedule and not the schedule of an organized tour.
2. You can make a detour without any problems and make stops along the way (not the case with an organized tour).
3. If there are two or more of you, it will be cheaper to rent a car than to pay for an organized tour. The approximate cost of a full-size rental car is about $35-40 per day + gas. The cost of a ‘half day’ trip to El Yunque rainforest from the Condado / Isla Verde area is about $45-50 per person. The cost for an organized trip to the Camay Caves or Ponce will more.
The main downside to having a rental car, is the driving in Puerto Rico. It is a challage at first (see this post), and the drive from the airport to the tourist areas can be very busy. But if you take it easy, you will be fine. Once you are away from the San Juan and tourist areas, the driving calms down.
One thing that I really highly recommend is that you check the prices for car rentals online for all of the major rental companies; as I have found that there can be a significant differnce in the daily rates for the same size car.
If I was visting Puerto Rico on vacation and wanted to see parts of the island away from the normal tourist area and San Juan, I would definitely consider a car rental. You just have so much more freedom to enjoy Puerto Rico.