Positive: the location is pretty good. Tram and metro stations very close. The flat is indeed at walking distance to the Alexanderplatz. The flat is spacious, with 3 different rooms (bedroom, small living room and big kitchen) plus quite a generous bathroom. All renovated and confortable. the check-in was smooth, Lucas gave me all sort of instructions upon arrival.
Negative: No elevator and the wifi continuously drops (on top of that you have to sign up using login and password every time it drops...) Then, something really minor, but basic things like a hand towel, gel/champoo, real knifes and pans or kitchen towels are missing. This would make the stay really complete.
Overall, a nice experience. Recommended.