Markus' apartment it's just great. It's very well located, with tons of restaurants around and super close to the metro and many other buses (included the ones going to Schönefeld airport. The area is vibrant but remains quiet during the night.
Markus communicated very quickly as well, and the checkin was super smooth.
The room itself it's big and very light and the bed is quite comfortable. Also, the windows give you a great view over the city. The apartment is very clean too. Keep in mind it's a top floor without elevator, so if you're carrying heavy luggage, be ready.
The apartment is very close to Tempelhof Airport (very recommended to stroll around) and within walking distance to Kreuzberg.
I'd totally recommended it.