A quick review in two parts - Stefan as host and the location of the apartment.
Stefan is a great host. He is very flexible...I arrived in Berlin at 10:15, and could check-in immediately instead of the advertised 18:00 check-in time. Stefan has great English and was patient enough to allow me try my German. We were chatting away one evening and he offered to share his meal with me, which I thought was a very generous thing to do. It is not something I have experienced with other hosts.
The location of the apartment is fantastic. It is on a quiet street, which also has a bakery and restaurant, so if you don't feel like cooking you don't have to go far to get great food. There are 2 U-bahn (underground) stations within a 700m radius and the Bundestag, Brandenburger Tor, Schloss Bellevue are easily reachable on foot.
If returning to Berlin I would certainly check out availability at Stefan's first. Great guy, great place.