I was there with the family for two weeks and a car would have been absolutely unnecessary. Taxis are plentiful and inexpensive (super inexpensive if you want) and local transportation is absurdly inexpensive (US$.30 from the resorts into town!). I didn't meet a single fellow guest who rented a car. That being said, there is an Avis on site in case you want to take a drive to the far sides of the island.

Most of the island concessions (scuba, tour company, activity companies) will arrange to either pick you up at the hotel or they will pay for your r/t transportation. We booked our scuba dives with a dive shop in Nadi town and they personally picked us up every morning and then paid for a taxi to take us back when we were done.

Our AuPair booked an adventure tour with a company that was a couple of hours away. The company paid for a taxi to come get her and to take her back.