The room itself is nice and cozy. It has everything you need and there's a subway station and supermarket very close so it's easy to reach Berlin's central area. Johannes gave us a very nice, friendly welcome and explained everything well.
The only small minor issues with the apartment were that the ventilation in the shower wasn't powerful enough so it got hot and humid very quickly and the absence of an airco so the rest of the apartment was also quite hot (we knew this in advance from the ad). We had to clean kitchenware like glasses, plates and cutlery before use too.
Unfortunately my friend got sick and therefore we lost a duvet cover and sheet. To our surprise Johannes and Annika wanted to charge us a greedy 95 euros for 2 duvet covers, 2 bed sheets and 2 tea towels. We didn't have any tea towels to start with and only lost 1 cover and sheet. After offering a fair sum of money to replace what was actually lost, we never got a message from them anymore.
I would recommend the room, the stay was very nice and you will have a good time, but the greediness afterwards was a big disappointment for me.