The location is perfectly central (150 mts from Hackescher market and 600 mts from Alexanderplatz, so close to restaurants, shops, museums, transports). The apartment isn't big but there is plenty of space for a couple. It is very bright (9th floor) and functional. Moreover, we liked the view, especially at night (the Berlin TV tower's light games).
We didn't meet frank - his housekeeper welcomed us - but we stayed in contact with him during our vacation.
We definitely recommend it.