Although a car may be more costly, if you're traveling to the countryside and smaller towns and villages, it's probably the best way to go. Gas prices around my area range from 1.34-1.37 euro per liter. Diesel is about 20 cents less and diesel cars usually get better mileage so you could save a lot by getting a diesel car. That's what I do.
I also lease a car since I'll be here for more than three weeks. Car leases or buy backs are a good deal for longer periods of time. There are no extra hidden costs and the insurance is fully included in the price. However, if you pick up your car outside of France, there's an extra charge and you can only pick up at the Rome and Milan Airports. This year I'm leasing a Peugeot from Auto Europe (Buy Direct). They also have car rentals and even Vespa motor scooters in Rome. I've used Renault Eurodrive (buy direct) in the past, too.