We had a great stay at Rainer's apartment in Berlin! The location is very near Alexanderplatz, which is perfect if you're arriving at the main train station: it's just three stops from Berlin Hbf to Alexanderplatz and about a 6-minute walk from the Alexanderplatz station to the apartment. Very convenient for our group, especially after a long train ride from Amsterdam.
Rainier was easy to communicate with and even let us leave our bags when we arrived, even though the housekeeper wasn't quite finished yet. The apartment was well equipped and nicely furnished. My only suggestions for Rainer are to fix the washer (it was out of order and we had to do our laundry in the sink), ask his housekeeper to give the shower and tub a deep-clean soon (the tub was mildew-y) and possibly update the Wifi router. We were able to connect to the internet, but it was slow for four people with separate devices.
Overall, we had a wonderful stay in Berlin at this great value apartment and these things are just small suggestions for improvement - thank you for having us!