I stayed at Carsten's property for one week in July 2021. It was a great find. Located near Suedkreuz Hbf (approx 8-10 min walk), the apartment is located in a quiet neighborhood. It offers good connections to downtown - Julius Leber Bruecke Station is approx 8-10min walk away too offering alternative S-Bahn connections and the bus stop a few blocks away offers connections to Kurfurstendamm and Zoo (I think the route is the 204).
The property itself is well kitted out - good shower and bathroom with plenty of towels and toiletries provided, decent kitchen with all the amenities necessary for self-cooking, plenty of closet space, flatscreen TV, etc... The big pluses of the apartment for me were the excellent air conditioning system (crucial for those hot Berlin summer days), and the cute terrace, which not only provided a pleasant view of Berlin but was nicely decorated with plants and flowers, good furniture, and was very private - the perfect place to come back and unwind after a day out and about in Berlin. A small drawback for some might be the apartment's location on the fifth floor (which in fairness is clearly advertised). As there is no elevator, this could be a challenge for those with mobility issues or with lots of luggage. Nonetheless, this is offset by the advantages mentioned above.
Finally, Carsten was entirely professional throughout - he was super responsive to my correspondences; upon arrival helped me up the stairs with my excessive luggage and provided a clear tour of the apartment. He also left a bottle of wine and some chocolate as a welcome gift which was a nice touch. I would have no hesitation in recommending the property for others - suited best me thinks to a couple or solo travelers and would willingly stay there again.