Katrin's apartment is fantastic. Very comfortable for up to 5 people, small but super functional, it has anything you need. It was very clean, the beds were comfortable, the shower worked flawlessly... all great.
The area where the apartment is located we found it very nice, surprisingly calm, everyone we interacted with was very friendly (don't miss out the opportunity of spending a relaxing time at night at the shisha bar next to the building). The area is very well communicated by public transport with the rest of the city.
In all, fully recommendable, and would return again on my next visit to Berlin!