Amazing location (restaurants, train station right outside the window, Alexanderplatz five minute walk away, all the attractions of Berlin only a few minutes away by train). Nice spacious apartment (large rooms, fun kitchen & dining area, great balcony overlooking Hackescher Markt). Really nice hosts who helped us out with a crisis towards the end of our stay.
Only downside was the smell of sewage in the entrance to the building, but that's not the host's fault. All in all I highly recommend staying with Denis & Peter.