Eric's apartment is a wonderful, character driven space in the heart of the Marais. The location is superb for active folks, with only one caveat: this is a 4th floor walk up, which is not an issue if you're an able bodied adult but may pose challenges if you have heavy luggage or a cranky kid.
Overall we loved our stay: easy access to Place du Vosges, Ile de la Cite, Notre Dame, the left bank - everything is within walking distance, and the Metro station, just outside the door, connects you throughout the city.
If you visit, be sure to check out the Bastille Market on Thursdays and Sundays; rue de la Roquette around the corner is full of cafes, bistros, and a grocery store (monoprix).
Be aware that the front room, with a twin bed, does stay a little noisy throughout the night due to traffic on the street below; the inner bedroom, queen size bed, is silent and peaceful, and the living room is quite comfortable.
The bathtub is perfect at the end of a long day of sightseeing. You may want to consider bringing some epsom salts or bubble bath if relaxing in the tub is your thing!