Sebastian is a friendly, tolerant, positive and enthusiastic host - and is flat is very cool. Do not be put off by the roadside entrance (very unprepossessing). You walk through two courtyards thinking "have we got the right place??" then mount a beautiful wood-tread staircase. Then your hopes rise, but when you enter the flat - wow.
easy to get from there to the rest of Berlin and great fun base. Sebastian's pre-loaded fridge with drinks etc is great, and we all had a great time.
We did ask for two extra mattresses (2 extra guests) and felt we were charged quite a lot for this, so if you want more than the stated number, maybe agree this first.