The apartment is nice on the second floor of an old building(no elevator) about 6-7 minutes walk from Hermannplatz Metro station. We never met Claudius but his friend warmly welcomed us upon our arrival. Although the check-in time was at 5 but he was nice enough to give us the keys earlier at noon.
However, the feeling was like staying in a small hostel or a guesthouse. There are 3 bedrooms for 3 groups of guests, a shared bathroom, a shared W.C and a shared small kitchenette. We never had the chance to communicate with someone during our stay. The furniture and facilities are very basic and generally it doesn't look like someone's place.