I had a fantastic 10-day stay at Denis's apartment, centrally located in Le Marais (4eme) near the Saint Paul Metro. It was the perfect spot for an extended stay, as easy to get anywhere in the city, and the Marais itself has everything you could need as far as great restaurants, cafes, shopping, nightlife, and history (it's one of the oldest parts of Paris). Denis himself was very communicative and even bought some groceries for my breakfast the first few days. The apartment itself is very stylish, has all the amenities you'll need (including a washer and N'espresso machine). I stayed in January, and heat worked well too. I'd definitely stay here again.