Karsten's place is quite nice, in a quiet part of Mitte, *very* close to U-bahn. It was easy to get to Kreuzberg, Tiergarten, and even Kindl-Bühne.
Karsten spent a few minutes with us to share some of his favorites spots, on the map he provided for us. Very friendly and helpful!
One thing to note, the flat is on the bottom floor, and on the corner, so there can be a lot of street noise, and noise from neighbors entering the building. This didn't bother us, but may bother others, especially late at night.
All in all, we loved the place. Would recommend it, and would consider staying there again.