I enjoyed my stay at Markus's home!
I would have liked to get the chance to talk to him more as he seemed like an interesting guy, but I was out most of the time so only really saw him at check-in. However, the apartment was very clean and in a very cool part of Berlin near Kreuzberg, with tons of good cafes and bars nearby. Although Berlin is huge, most of town was easily accessible by bike or public transport, and getting there from the airport was straightforward. As a bonus, Markus also let me use his laundry machine.
My one complaint is that the room had no window shades, and the sun in Berlin rises before 5am at this time of year (May), which meant the room was very bright when sleeping. I can sleep almost anywhere, so it worked out fine for me but wanted to warn any sensitive sleepers!