The three of us stayed in Romain's place over a long weekend. We greatly enjoyed our stay and can certainly recommend the place.
The area of Buttes aux Cailles is lovely and authentic, with numerous bars and restaurants; several metro lines are within 10 min walk.
The apartment is exactly as described, spacious and modern, with all the necessary utilities, good heating, good WiFi signal, etc. It's not very sound-proof though, so bring your ear-plugs if you're a light sleeper.
Romain has been extremely responsive and helpful. The check-in and -out were easy.