Communicating with the owner was easy and the apartment was spacious. There was plenty of room for 6 persons. Also, the living room and the kitchen with a long table and basic thing for cooking were good. The location near two U-Bahn stations is excellent. And the surroundings are very tranquil (German´s military and secret service are in the same quarter).
It might be a good idea to be in touch with the owner a night before check-in just to check that the key for the apartment is really where it should be (in a locker outside the building) and the apartment is clean. In our case we arrived late and the key was not in the locker and the apartment was uncleaned. After several attempts we managed to contact the owner by and she alarmed cleaners. After almost 2 hours of waiting we were finally in bed.
The owner apologized the delay and offered kindly to compensate the amount of cleaning (95 euros), but so far I have not seen the money on my bank account.
A few days before our check-in the owner asked me to pay Berlin accomodation tax (5% of the total net amount of the stay) to her private bank account. I asked 3 times for a receipt for the payment, but never received it. So it is hard to say, if the payment really went to Berlin tax authorities or somewhere else...