Guillaume's apartment was a fantastic base for us in Paris. We were there for a month and really appreciated the central location, convenient layout, and space to have guests crash for a few nights on the couch. Having stayed at a number of airbnb apartments in Paris around this price range, I think Guillaume's place is a real deal! I'd certainly love to stay there again if the apartment is available on a future trip. As for Guillaume, he's a great, communicative host, who really cares that people have a good stay.
One thing to note: the living room is a few floors above a 24-hour restaurant, which means it can be tough for guests to sleep with the windows open out there without earplugs (the bedroom is in the back and totally fine!). With the windows closed (tough in summer) it's also fine.