I stayed at Martin's house for a month and I really loved it! It's in the greatest location possible, near to the best coffee houses and bars, and the very beautiful Landwehrkanal. Public transportation is right next to the house, so you can get anywhere in the city very fast (most central places are 15 min away).
The room is spacious, nicely decorated and very comfortable. It has a lovely little balcony with a view to a beautiful yard. Even though in a very popular area, the street is actually quiet, so the house is quiet as well!
Martin is a very nice and cool person to live with, you will always hear nice stories from him and he can tell you a lot about Berlin and Germany (if you are a foreigner like me). He is extremely helpful and will try to accommodate you the best way possible during your stay.