Wine bars with a terrace aren't that many in Paris, you have Aux Tonneaux des Halles rue Montorgueil or Le Mauzac also near the Panthéon and otherwise it's often only a couple of tables and that's it.
The Café de la Nouvelle Mairie has also a terrace, it is heated in winter and overlooks a tiny square on the side with a few trees and a fountain (see the picture on the right with the Café de la Nouvelle Mairie in the background), a stone throw from the Panthéon, one of the main historic landmarks of Paris (a church turned into a secular temple under the French revolution). When the evening comes, the street and the square get even quieter, which makes the place very attractive when you think that you're in the middle of historic Paris.
The clientèle of this wine bar is very diverse, they can be artists of the music industry (Universal Records is on the other side of the street), students, teachers (there are many university institutions around there like the sorbonne or the Institut Pierre & Marie Curie), professionals, bobos and locals, the latter coming often in the evening after work. As Benjamin (the manager) says it, it is a wealthy neighborhood but with middle class people too. It's not the 7th or the 16th, the place has a soul and breathes intellectual life.
If you're looking for ambiance in the latin quarter and want to drink artisan and natural wines, that's certainly one of the Paris wine bars to go to.
The photographs were by Manu Chavassieux, see his wine-people pics here, you'll recognize many key people of the wine bar scene. Many of the people featured on his close-range black and white portraits were around that evening. His website name (Nikon ni Soumise, sounds like neither dumb nor slave in French) is a humoristic allusion to Ni Putes ni Soumises (neither whores nor slaves), a support group set up in the muslim suburbs to help secular, unveiled women organize and resist threats and violence because of their independant way of life.
I know a few of these pictured people, but not all of them by far, I think I recognized a few faces that I remember having come across at tastings here and there. The exhibition opening started slowly by 7pm and then the room and the terrace filled at a point that you had almost to stand on the street with your glass.
The whites will be bottled next summer, in august, and available in september. He sells half of his wines in Paris, in la Nouvelle Mairie of course, but also at Le Verre Volé, at Le Baratin, Caves de l'Insolite, L'Amuse Vin and other cavistes dealing with natural wines. His prices are something like 6,8 € without tax (for professionals) and end up at about 13 € for the reds. For the whites, the pro price is 9 € and retail is 18 €. He exports already, to Japan, to David Lillie in New York (Chambers Street Wines), London, Geneva.
They also keep a limited list of the wines in bottle "that taste well" : about 13 bottles are listed on a blackboard ("les beaux flacons", priced from 35 € to 90 €) with for example a Pierre Beaugé (Auvergne) vin de table "Vitriol" 2006 at 35 €, a Bandol Saint Anne "Collection" 2000 at 59 €, an Overnoy Arbois Pupillin Chardonnay 2004 at 35 €, or a Fleurie 2006 Ultime by Yvon Métras at 45 €.
To go with these wines, you can order some food (not an obligation, it's a wine bar) and la Nouvelle Mairie is also known for the quality of the solid food. The charcuterie and vegetables, meat and everything is sourced directly in the regions. Prices begin at 5,9 € for a planchette of charcuterie.