We stayed for a week with 4 adults and a 1 year old child. Communication with Airin, our host, was excellent and we found the apartment easily with the detailed directions she sent us. The apartment is genuinely Japanese (beautiful, clean, small, high utility), clean and has everything you need to feel at home. The included portable wifi hotspot was very useful and makes this place quite a bargain. Airin also lives nearby and goes out of her way to help. It was very quiet. We absolutely recommend the apartment for up to 3 people. We also recommend it for 4, but only three futons fit in the tatami room and storage space may become rather limited.
We also recommend staying with an infant/small child. However, some things on the floor may need to be put away out of reach (bin, plant, maybe TV, cleaning utensils). There is a nice green path leading in the direction of the station with no cars and two playgrounds (Setagaya-Maruyama Park) plus a small public fitness station(by Renaissance Sports Club Sangenjaya) which is nice for walks or runs. We also recommend the small wooden toys shop on Setagaya-dori (Sangenjaya, 2-chōme−17−11, I think) which is also a cafe / ice cream shop and has a little play area where almost every toy can be tested freely!