I stayed at the apartment for a whole month, and had a wonderful experience. The combined living room/bedroom is nice and large, and the kitchen and bathroom have adequate space. Sunshine in the morning lights up the living area. I used the patio for breakfast on warm sunny days. The whole apartment has a very nice contemporary sense of style. Katja and Bernd bend over backwards to answer your questions via email (or phone). I was very pleased with their graciousness and hospitality.
The apartment is within a five minute walk to the metro (U and S Bahn, tram) and major shopping arcade, as well as bars/restaurants. Perfect location from my perspective - I loved Prenzlauer Berg. The only thing to be aware of is that the apartment is on the third (US - fourth) floor with no elevator. Great exercise for me, I had no problem with it.

I highly recommend the apartment.