I spent a whole week in Frederic s appartment. It was a clean, very good flat in good conditions and very well ubicated. His father received me and gave me the keys (who turned out to be an Argentinean too :-) ), and gave me a few references about the neighborhood.
Metro Plaisance (L13) is a few mts away, so u probably get anywhere in paris in 30 min. Except if u have to go to Pere Lachaise or Parc de la Villete, that is in the other side of Paris.
The appartment has some sofas, chairs and tables, so it s very handy if u have to work with ur laptop maybe.
The neighborhood is very cool, restaurants are cheap and really good. Sometimes it can be a little bit noisy at night but totally OK.
I would recomend to stay in Frederic's appartment.