We had a great time for 2 days at Frank's place.
The location is great, very close to Alexanderplatz and metro station etc.
Frank is a nice guy, was very friendly and flexible with check in and check out time. He gave us some good tips for restaurants and bars around the neighborhood, as well as a city map.
The studio could have been a bit cleaner and the curtains a bit thicker (the sun light was a bit annoying during morning sleep after clubbing) but overall it was a great experience and I would recommend Frank and his place for a nice stay in Berlin!