As a family with small children visiting Berlin we loved the apartment and the area. Modern, light and clean, and with a great backyard with playground and tabletennis. Placed in a quiet and calm street, but still very central to right next to Alexanderplatz, and just a minute walking to cafees and restaurants etc.
Never met Rainer, but communication through airbnb was very good, and he was very helpful with preparing, parking etc. and made us feel welcome with beers in the fridge.
All in all we highly recommend Rainers apartment to everyone wanting a nice and central base during their stay in Berlin!