We had a great stay at Martin's appartment! The appartment is located in - in my opinion - one of the best areas of Berlin. It is near the Bergmanstrasse which is a very cosy and lively street during the afternoon and night. The U-bahn is a 5 minute walk but we mainly used rental bikes (situated a 3 minute walk away).
The appartment itself has all you need. A good warm shower and good beds as well. I would recommend considering bringing a sleepmask when you want your room to be completely dark at night since the blinding doesn't keep all the light out (not a huge problem though, just so you know).
It was really easy to get in contact with Martin, the self check-in went flawless. He also told us where to park the car for free just a few minutes away.
All in all we had a great stay, I would definitely recommend staying at Martin's. If we will visit Berlin in the future we would stay here again.