I lived in Mende as an English Language Assistant for a school year and Les Voutes was one of the best restaurants in town. The restaurant is the interior of what appears to be a hallowed-out cave (from whence it derives its name), so there are no windows, but the walls are velvety grey stone and, as I recall, there are candles on the tables, giving it a very nice ambiance.
The food was quite reasonable, provided you do not go all out (apertifs, salad, entree, cheese course, dessert, coffee...) If you are traveling on a budget, stick to a basic entree and dessert, and simply drink water with your meal, and you will enjoy yourself for under $20. They serve pizza (french style, unfortunately: with Emmental cheese instead of Mozzarella) and pastas with meat sauces, all of which are very good, and their desserts are amazing-- the chocolate ice cream is the actual color of a dark chocolate candy bar (note to UK readers: here in the US, our chocolate ice cream is a pale light brown) and agonizingly good. They feature ice cream sundaes with clever names, none of course which I can remember now, topped with mountains of chantilly (whipped cream).
Les Voutes is really popular with the university students in town (there are at least two schools-- one for travel and tourism, and one for teacher training-- in Mende that I know of, so there are quite a few twentysomethings in what you would assume would be an otherwise sleepy town). Friday and Saturday nights, the place would be packed. They opened for dinner at 7pm, and it was necessary to arrive there as soon as they opened, so as to be finished by the time the place was packed and so full of cigarette smoke that the air was foggy. However, now with France having passed and (supposedly) enforcing a smoking ban, hopefully this will no longer be a drawback, which was the sole con in a long list of pros for this place. Definitely go!