We stayed in Karstens apartment with my friend for 5 days. We had an awesome time in Berlin, and we really did enjoy staying in this apartment. It looks exactly same as in the pictures and the location was great!
Karsten was a good host, although he was out of town when we arrived (really early in the morning) we were able to go straight to the apartment because Karsten had come up with a good arrangement for us:)
I would definitely recommed staying in Karsten apartment in Berlin!