We were three couples, one with a 2 year old child and we were very happy with the apartment. It is very close to a number of bars and restaurants along the river and the shops of Friedrichstrasse, so the location was excellent.
On arrival, Maria was there to meet us and was very welcoming. She showed us around the apartment and how to operate various appliances/keys etc.
The apartment itself is very spacious, clean and comfortable. The inclusion of the washing machine was very useful as we stayed 6 nights so needed to do some laundry. It seems that the floor below is occupied by an office so there was little or no noise - sometimes a worry when staying in an apartment. The beds and bedding were clean and comfortable. There is only one bathroom, though it does have a shower and a bath. A separate WC is also handy.
The apartment is up several flights of stairs though a lift is available (though there is still a flight of stairs to climb, even after using the lift).
All in all, very happy with the apartment and our host Maria. Most of all though we loved Berlin - a wonderful city. We will be back!