We didn't meet Katrin, but we loved her studio apartment. The apartment was bright and clean, just as shown on the photos. The check in/check out process is very simple and time-saving. The neighborhood is residential so it's quiet during the night, but there are many coffee shops, bars and restaurants in the vicinity so it's the perfect location for your stay in Berlin.
Mind the fact that the apartment is located on the ground floor and the tram M13 runs in front of the building so that might be a problem for you if you are a light sleeper.
Overall, I would recommend Katrin's place for young people since it's in a hip neighborhood in East Berlin.