Matthieu's place was mostly as advertised. It is a small, studio apartment near a trendy part of the city, with easy access to several metro lines. It's just northeast of the 3rd arrondismont, which makes it a half hour walk from the Seine and the Notre Dame. It's a good place to spend a night or two.
Matthieu was not able to meet me to transfer the keys to me, as per the initial check-in instructions. After that, the messages back and forth were unclear, missing information that should have been included, or lost in translation. For example, he mentioned that he would leave the keys downstairs, but he didn't indicate with who. After asking for more info, it still wasn't clear. It wasn't until I asked again after arriving that he told me to go to another entrance and ring the building guard office and figure out where to go that I was able to get into the place.
As mentioned, the place is in the city, close to downtown, so expect things like that from the area. As the other review mentioned, there were some loud drunk folks outside; no big deal, but as the place is on the first floor, keep that in mind if you are a light sleeper. And there were a few homeless people sleeping by the building entrances.
That said, the place was mostly clean, and the sofa bed fairly comfortable. For the cost, it's definitely worth staying a night or two.