The best way to visit the island for a day. There is no meter, so a little bit of haggling won't hurt ; otherwise you always have the option of booking your taxis via your Tour Operator (who will negotiate the prices directly with the taxis firms).
Our opinion : you won't get lost, you will be driven all around the island and if your driver is very chatty you could learn some magnificent details about Mauritius and its inhabitants.
Some prices indications in rupees (2004)
Airport > Grand Baie (one way)
All day long > island tour
Grand Baie > Port Louis (one way)
Grand Baie > l'Ile aux cerfs (all day long )