Birgit and Hassan are both very nice and welcoming people and I can only recommend staying with them.
I spent 1 week in Berlin on a work trip and being able to stay at their place was like having a home away from home and made the whole week less stressful. The room is even bigger than it seems on the pictures, it is super clean and I wasn't missing anything. In the morning they always had a little surprise for breakfast, which I really appreciated. It is located very conveniently close to the U-Bahn Kottbusser Tor in Kreuzberg, so it is in a very vibrant part of the city, but still quiet enough that you can sleep very well.
I am really happy that I stayed at Birgit's place and got to know her and I would immediately return the next time I'll be in Berlin.