Rent in St. Thomas instead of St. John ($$$$)
Pros: SUPER value, especially as compared with rates on St. John. Renting on St. Thomas and paying the $50 round trip on the ferry is DEFINITELY the way to go.
I rented at the office in Charlotte Amalie, which is located outside of the airport, and the cost was a lot lower. Despite what the website said, they allowed me to return it to the airport for no extra cost.
Staff was extremely friendly and helpful.
Cons: St. John's roads are insanely steep and curvy. Transmission was not in great shape, and the car really strained, sometimes coming to near stops on steep inclines, even in low gear. Given my experience, I'd opt for a Jeep type next time.