This is a great value: basic accommodations at a low price. The room is plain (bed, desk, wardrobe, lamp, radio), but it is clean and comfortable. There is a lavatory solely for the guest, with the bath shared. The flat is in a quiet street in a lively but safe neighborhood. One must walk up four flights of stairs to reach it.
The building is a 7-minute walk from the Gneisenaustraße U-Bahn station, and 6 minutes from the Platz der Luftbrücke station. There is a laundromat across from the Platz der Luftbrücke station.
The host is knowledgeable and helpful, and he knows the neighborhood well. He is fluent in German, Spanish, and English, and he understands Italian and Portuguese. He recommended a good bicycle rental shop to me just a few minutes' walk from the flat and also helped me find a hotel near the airport for my return trip to Berlin.