We had originally planned to only rent a car for one day, from Ste Anne where we were staying, but when we got there and realized the cab fare to Ste Anne was going to be $50 we changed our minds and rented a car from Hertz for about the same price, $45 per day. We rented a standard (manual), as automatic transmissions are hard to find, and much more expensive.
The roads in Martinique are excellent. They are well paved, extremely well signed, and drivers are not overly aggressive. The only problem we ran into was that some of the towns have very narrow streets, which makes for some interesting parking, and last-minute breaking for pedestrians, who regularly step out right in front of cars without even thinking about it. For people unaccustomed to standard transmissions, constant hill starts when driving through the bumper to bumper traffic on the highway going through Fort de France can get quite tiring.