we like to hire a driver for the day, that way you get a built-in tour guide to drive you around the islands - you can usually hire a driver for a few hours for a less than the cost & worry of a rental .
When we went to St Thomas we took the red hook ferry over to St John and rented a jeep for the day. We wanted to explore the island and go at our own pace. It was truely the decision we made! Very bueatiful over there and we were able to go around the entire island ourselves! And the jeep was about 60.00 for the day too which was more then 2 shore excursions on our ship would have cost. We went to Trunk Bay, the Annaberg Sugar mill ruins, and ate at Skinny Legs (really yummy hamburgers!).
Unless you're used to driving on the left side of the road you would be better off in a cab.
The few American tourists we saw driving were having problems in traffic. Plus you have to worry about damage to the vehicle while you have it parked. Your U.S. automobile insurance doesn't cover any vehicle damage abroad.
There are stories on the message boards about people driving jeeps in Barbados through the sugar cane fields getting stuck on large rocks, then being robbed by the locals. I can only say that I saw men in the sugar cane fields carrying machetes and was glad to be in a tour group.
Definitely NOT true! Our automobile policy (USAA) DID cover accidents abroad. We even called them to confirm this. And driving on the left side of the road is NOT hard. The only bad thing is the roads are windy and hilly but other then that it was very easy.
And going OFF road is a stupid thing. Stay on the one main road and you'll be fine in St John. It goes completely around the island.