Hi,
You can hire a car if you want too, but Louages are a very popular means of travel in Tunisia because they are cheap and they can take you practically anywhere! You can get a louage from Monastir - Tunis, Sousse - Tunis, Hammamet / Sfax - Tunis and the list goes on, there are louage stations in lots of places and also louages act as normal taxi's in the local areas running you from place to place. A louage from Sousse to Tunis costs 8 dinar per person, and for example if you were going from Kantaoui to Sousse (local areas) about 500 millimes!! Cheaper than a taxi by all means.
They even come in handy for when moving house!! we havent got a car yet, and weve recently moved from Sousse to Tunis, we rented a louage and the driver, he picked us up, loaded all our belongings and drove us to Tunis, right to our front door step.
Downfalls, can be smelly

you get approximately 8 people in each louage, can be stuffy in the summer, but they can be pretty nippy if you get a crazy driver.
Louages run at all hours, we even grabbed a louage at 2.00 am!