Hello,
I was in Madagascar last July for almost three weeks with my girlfriend. We spent quite a few days in Antananarivo trying to find a reasonable car rental service and ended up renting from Hertz.
We took the cheapest option (at about 45 euros per day), a rather new Peugeot 207, which proved to be very good on paved roads (avoiding the big holes here and there!) and also good on sandy roads, in Morondava and Ifaty, where we were not supposed to go, but we did, without problems.
The service from Hertz was friendly enough, we didn' t have any problem from them or from the car, only from some policeman asking for money in Tulear.
We never met any other self driving tourist; everybody had a driver, but I must say I really enjoyed driving in Madagascar (you need to be an experienced driver, though).
If you decide to look for other car rental (we couldn' t find anything else at a reasonable price, anyway), beware of a gentleman called "Philippe", who will be trying to rent you a piece of junk in the shape of a Renault Clio. Just a waste of time.
If you have any other question, just let me know.