Fares on OUIGO, the budget 'fast' train service offshoot from SNCF might start at 10 Euros yet when you take into consideration that inaugural service leaves from Marne-La-Vallee and not Paris itself and you obviously have to pay plus spend time to get there and they charge you to plug your laptop or cellphone and limit baggage you can take, OuiGo sounds to me like a No Go.
The train advantage on short to medium trips is that it allows you to travel city center to city center.
A better option with some of the same features (book online only and print your own ticket) is iTGV which I used in 2011 from Paris to Toulouse.
I just did a bit of comparison shopping for travel on May 15, 2013.
Using OUIGO from Marne La Vallee to Marseille St Charles on that day best fare quoted is 30 Euros, leaving at 12:22 pm, arriving at 15:34.
Using iTGV from Paris Gare de Lyon to Marseille St Charles on same day best fare quoted is 35 Euros, leaving at 15:07 pm, arriving at 18:24 pm.
Even regular TGV offers great rates if you can plan ahead.
Besides Marseille, Ouigo also currently serves Lyon and Montpellier.
(* Ouigo image from Ouigo website)