Our stay in Wendelin's flat went incredibly smoothly. The apartment is exactly as described, its location is brilliant (it's on a quiet street, across from a Hofer branch, making cooking really easy, and is a stone's throw from one of the central U-Bahn lines). It's very clean, and had all the amenities we needed. Wendelin is a friendly host, he was quick and helpful with his replies, and waited for us patiently at the flat when we were slightly late with our arrival. Positive impressions all around!
The only point of improvement would be regarding luggage storage. The flat did not include a luggage storage option, which we would have really appreciated since we left the city at 9 pm, but luckily, storage at Hauptbahnhof is very reasonable (€3.50 for 24 hours). Considering the fantastic value of the apartment, this was really not a big expense to spare, but could be worthwhile to keep in mind. :)