Most Japanese hotel rooms are tiny. At MyStays Kamata it says 15m2 in the room description. This might be true when measuring on the "outside" but it doesn't feel like 3m x 5m of usable floor space. Oh well, at least the room was large enough to move around, and store 2 suitcases. I have seen rooms far more cramped, so overall for 3 nights it was fairly OK.
WiFi is stable and fast. Passwords for each individual room with good log-in instructions. WiFi system did remember the devices for 3 consecutive days. Nice, this is not always the case elsewhere.
Ice machines on 5F and 10F. In good working condition. Also soft drink vending machine.
Only one elevator for whole building, can get busy at peak times but hey, floor space is valuable.
Amenities provided in bathroom or lobby. Good shampoo and conditioner. Good soap.
Free freshly brewed coffee in lobby, if you ask for it.
30 meters across the street is not only a large Marutetsu supermarket (1F) but a Daiso 100 Yen store in the same building (3F).
Rooms are made up when staying 2 nights or more. Make sure to place the magnetic sign on the door.
Cleanliness and design very attractive and functional.
Overall: Recommended without hesitation. Stay here if you are on a budget, don't need breakfast and don't mind the train rides. 11 minutes to Shinagawa with blue Keihin-Tohuku line.