It's a wonderful 2-floor apartment with a separate entrance and a communal yard. It feels more like a small home. The apartment is well-equipped and has all the facilities a group of friends or a family may need. It looks like the bathroom has been recently renovated and everything works very well. Rooms are rather small, but there are enough space for 3 or 4 people.
The location (Belleville) is awesome. There is a metro station in 5 minutes walk distance, and it takes less than half an hour to get to the city centre (Louvre, Notre Dame, Centre Pompidou etc). The quarter is very lively and there are great diversity of places to go to, like hip coffee places, bars, small private restaurants with French, Vietnamese, Arabic or African kitchen. Locals call Bellville "bobo", which means bourgeois bohemians, and it certainly has something of a bohemian atmosphere, but it's still cheap and cool.
The building itself is a former fish store, and it probably explains increased humidity inside, which is felt when it's raining. It's certainly not unbearable, at least for me, but if you're sensitive to humidity, take note. But I don't think it'd bother you if you go to Paris in the summer.