Valentine's flat is perfectly situated between the Gare de Nord and Gare de l'Est train stations, as well as other metro and bus stations. It is located in a very lively area with a variety of different bars, restaurants, cafes, shops and you don't need longer than 30 minutes to get to Paris biggest sights. Valentine was so kind to wait for us when we checked in, as our train had a delay in the evening and always replied to all our requests promptly.
It was a pleasure staying at her place and I would recommend it to all of the fellow travelers out there, who are looking to explore Paris on their own (Valentine also got us a list of recommendations and ideas about the neighborhood and Paris in general!) and to live like a Parisian for a while! :)