We are a family of 4 and stayed at Kerstin's flat for 2 nights. Hinrich, a neighbour in the same building, was in as arranged to handover the key and was friendly and helpful. The flat itself was spacious and very stylish and modern. It was nice and clean and plenty of bedding and towels were laid out. We enjoyed staying there and had no problems. However, it might be less well suited to lighter sleepers. There are the usual city noises, which are fairly audible on the 1st floor in summer when you need some windows open and the hard surfaces do little to dampen them, though it helps that the bedrooms are at the back. Also, in the study area in which the sofa bed was set up for the 4th person there was a window with no blind or curtain. I expect we could have improvised something if this had been a real problem for our kids.
The location is great, about 10 mins from Schönhäuser Allee or Bornholmer Straße S-Bahn stations and there are plenty of cafes, corner shops and supermarkets. About 15 mins from the super-trendy heart of Prenzlauerberg.
We would be happy to come back next time we are in Berlin.