Great place. Satoshi (Ash) is very friendly and has many local tips. He also was extremely accommodating for my issues checking in (due to missing the last train in), and even took the initiative in contacting Airbnb to compensate me. The common room is quite a nice setup for meeting the fellow guests!
Amenities are what you'd expect for a guesthouse, and I must say the shower was really good. Capsule-style bed was comfortable, I had no complaints during my stay.
If you want to stay in a guesthouse near Shinagawa, I recommend here!