We stayed at Britta's for one month. Best thing of the apartment is the location. The U6 station is literally steps away, it's convenient for both Brandeburg Airport and city center (Mitte / Unter den Linden). It's a small flat but good for a couple. We were never cold, heating worked perfectly (November - between 8 and 0 degrees, so kind important). The apartment is quiet but also really close to Bergmannstraße, full of bars and services (pharmacy, drug store, supermarket). Self check-in/check-out is fast, although I would suggest re-phrasing the instructions on how to find the apartment (it's in the inner court of the building behind a door that kinda looks like an elevator door). It's a first floor apartment but it's safe because you are inside the building, far from the street (streets are pretty safe as well). Before we got to Berlin, communication was exclusive to the Air BnB chat and it was a little slow. But as soon as we got there and talked through Whatsapp, Britta was always responsive, kind and useful, she even sent someone at 10pm when we had an issue with the internet connection.
The only con is the moisture in the apartment which is unavoidable since we are talking about ground floor in a cold city. I am sure it's different in summertime. Also, cleaness could be improved. Overall, it was absolutely fine. Thanks for having us!