As a longtime Parisian and historian, now living in the US, I dare say Serge's place is the best to discover the genuine atmosphere of Paris, in a neighborhood where most French revolutions began in the 18th and 19th c. The place is very comfortable, secure, quiet, pristine, with outstanding amenities (the kitchen is great) -- but not really suited for more than a couple or an individual, as it is quite small (but never feels too cramped as the ceiling is high). One caveat though: the bed is set in a mezzanine accessible by a steep ladder.
The neighborhood is wonderful : excellent and no-frills restaurant around, the best and most diverse street food market in Paris at close walking distance (Marché d'Aligre), amazing bookstores, wine tasting places and cafés.
And then, if you wish to visit the historic center, you are 20mn away from the Bastille, the Marais, and the Place des Vosges. Getting there is a real treat: rue du Faubourg Saint-Antoine, recently renovated, is for sure one of the most beautiful, and history-laden 18th c. streets in Paris, where alternative means of transportation abound (walk, bicycles, scooters, buses...).
In brief: a wonderful place to stay and discover Paris, and a great host.