Always do your own tours if you can. Papetee - The old Le Truck system is being updated and replaced with Busscars. I wouldn't recommend taking Le Truck around the island if you have any sort of back problem. The seating is basically wooden benches that have been thown into the back of a truck. The Busscars are much more comfortable.
Go to the bookstore and check into Jan Price's Tahiti. She has tons of info on the routes and which bus you need to take to get where. The book is pricy, but it's one of the best books for the islands. new one should be out soon (Amazon.com has the best price if you want to buy a copy.).
Taxies are easy to find at the dock, fix your price up front to avoid some surprises. Take a circle island tour for a fun day. Just be sure that the company has a van/bus/car with functioning A/C.