Positives: great location if you plan to visit shops/restaurants and bars in the Shinjuku area. the flat is located in a very quiet, residential ally. about 15 min walk to the main metro station (Shinjuku). the flat has everything you might need during your stay. its small (smaller than the photos suggest), but big enough for two people. the Internet is fast enough to play Netflix movies. the aircon works. the host is very fast with her responses. there's a spare mattress to accommodate an extra person (or two).
Negatives: while the area is very quiet, the neighbour makes loud noises late at night. my companion found the noises scary. the flat faces a wall, so it's rather dark during the day. at night, however, the lamps on the wall opposite to the windows beam bright lights into the flat. the curtains are too thin to block the light. you might need an eye mask (you can get one at Muji) to sleep well. the bed is very hard (which may be a norm in Japan).
Overall, i recommend the place. if the host changes the curtains (an investment of several dollars), the key negative will disappear.