Alex's flat is absolutely cosy, we felt it was our home.
It's very close to a S-Bahn station and a supermarket (Lidl). It takes about 15 to get to the city-centre by S-Bahn.
We couldn't meet Alex but we met Mike who helped us the day we arrived giving us direccions and explaining us which S-Bahn we should take.
To sum up, if you are coming to Berlín I totally recommend you to book this place.