Martin's unit is an ingeniously redone storefront space, with security and privacy concerns alleviated by a mechanical grate one can put down over front doors and windows while sleeping or out for the day. The location in Kreuzberg is marvelous: if you're traveling from the central train station, the M41 bus (once you've found where it picks up outside the main complex: you'll likely need to ask information if you're new to Berlin) drops off virtually in front of your front door, and other U trains are within a 8 to 10 minute walk away. The neighborhood is a fantastic jumping-off point to get around the city, even late, as transit in Berlin is efficient at seeming all hours and the M41 is a very frequently-serviced route. Transit to the BER airport is also easy by transit, with only one transfer necessary to make from the M41 bus to the S train. In short: there is minimal lugging of your stuff around the city whether you are coming to or from the central train station or the BER airport.
The neighborhood is chock full of eating options of varying price ranges, several bakeries and cafes, plus groceries. The unit is outfitted with clean and well-working appliances: we ran dishes through the dishwasher, washed clothes (a large drying rack is available), and hotplates are available for more involved cooking. The beds are raised loft units both at the front and back of the unit, and while folks with any mobility issues might have trouble getting up and down the steep stairs to them, we had no problems. There are also a good number of hangers available for clothes, something I've learned is among the most precious commodities in units I've stayed in. The interior courtyard is gorgeous as well.
Martin was terrifically accommodating for our later-than-expected arrival and was very communicative, and answered questions very quickly. He also had prepared guides and brochures for suggested activities around town and on arrival pointed out some really helpful features of the neighborhood to get us acquainted. He also had keys for both of us staying in the unit.
We absolutely loved Berlin and hope to be back very soon. Thanks so much, Martin!