Our flight was delayed, so we reached the apartment almost a couple of hours later than estimated. Maria's friend (also called Maria!) let us in. She was warm and friendly and patiently answered our questions about the nearest U-Bahn stations and wifi, etc.
The apartment itself was clean and well-maintained and located in a very nice residential area with cafes & restaurants, pretty boutiques and a supermarket just a couple of minutes away. The metro was also a couple of minutes away.
It was quite hot in Berlin when we reached (mid-August) and the apartment was not equipped with air-conditioning or fans. But that's not really a problem, since most European apartments don't have air-conditioning. The temperature became much better in the evening when we threw open the windows.
All in all, a pretty reasonable stay for the price we paid!
Thanks again Maria!