I stayed for three weeks at Karsten's place in Prenzlauerberg in January. I have only compliment for Karsten and his apartment. When I arrived Karsten and his family was there welcoming me, and showing me everything needed at the apartment. The apartment is clean and spacious for myself alone. Internet, TV, washing machine, heating...etc., all worked perfectly. The furnishing are all comfortable and in good condition.
One of the best feature of the apartment is its location right in the heart of Prenzlauerberg, where there are countless cafes, restaurants, shops, and supermarkets nearby, in a neighborhood that is very safe and family oriented. Right down the street, there is a biweekly organic market for baked goods, grocery...etc. The U-bahn is about 5 minute walk away, and the walk to the tram is even shorter.
It's really not a negative, but the apartment is on the ground floor facing the courtyard, so it's very quiet, but in the winter when I was there, there is not a lot of sunlight, so it is perfect for sleeping.
If I ever had a chance again, I would love to stay at Karsten's apartment again, and would recommend it to anyone interested.