Short summary this is a perfect place to stay in Berlin.
The apartment is lovely, it has everything you need. The room is very spacious and a large bed and a desk. Lots of space for your belongings & clothes. If it's summer you'll want to use the fan I left in the room for sure as Berlin gets hot!
The location is fantastic, it's a quieter part of Berlin compared to the east side of the city but there are plenty of shops, bars and restaurants nearby and quick & easy access to the city center. Notable locations nearby are vagabond brewery and Schneeule brewery bar (go check that out for the best Berliner Weisse you will ever have). There is a large Netto supermarket 2 min walk on the same street. There are other supermarkets and smaller spati shops all around the area. Super close to Nauener Platz station which is an easy start to the journey of coming/going from the airport or to get around the city.
Martin & Julia are lovely people with some great stories. The jobs that they do have taken them all around the world so if you have a love of travel then you won't be without a great conversation.
If I was ever staying in Berlin again in future I would stay here 100%. I had a great time and not only did I get a great apartment to stay in, I left Berlin with new friends.