Our group of five (three adults, two toddlers) spent five nights in the flat and had a great time. The location is great: 5-10 minutes to the metro, two grocery stores under the flat, 5 minute walk to the Luxembourg Gardens, and 5-15 minute walk to great shops and restaurants. The lift in the building was an added bonus: the flat is on the 1st floor (2nd floor to Americans) so not very necessary, but was helpful for our group of toddlers + strollers. The flat is clean and quiet; although it faces a busy street, it is not too noisy inside the flat, especially in the inner bedroom that faces the courtyard. Wifi was generally good, even with three people using it at the same time.
The only drawback for us was the lack of fans in the flat. We happened to be in Paris during one of the hottest weeks of the summer; while we were out for most of the day, the flat would remain hot through the night and opening the windows didn't help, making the nights extremely uncomfortable. For 95% of the year, this wouldn't be a problem but during the few days of the year when it's hot in the city, this may be an issue for some renters.
Guillaume is a great host: very responsive, easy check in, left us some maps of Paris, and even made dinner reservations for us due to our lack of French speaking skills.