SNCF is adding Ouigo to its portfolio of trains with services between the Paris area and Marseille and Montpellier starting on April 2. These will use the high speed tracks and thus have the same journey times as regular TGVs, but with lower fares, from 10 euros. They will be usin duplex TGV units that have been re-arranged with no catering facilties, reduced luggage space and single class seating. Booking will be only on-line (www.oiugo.com) and there will be a charge for more than one pieces of luggage. The "Paris" terminus wille be Marne-la-Vallee. There will be a 30 minute check-in requirement.
It obviously has some similarity with the idTGV service which may mean that US would-be users encounter difficulties with booking - it would be good to hear.
In general it may be worth trying.