Michael's place is an excellent place to stay during your trip in Berlin. The flat is large, with plenty of room. Both the bed and couch are comfortable, and a guest could easily sleep on the couch. The outside balcony is a nice extra, and is great for a meal or beer, or just hanging out and relaxing when the weathers nice. Kitchen has everything you would need if you wanted to cook. There's also an elevator, so no need to worry about hiking up a ton of stairs if you have luggage.
The flat is located close (8min walk) to a major U and S-Bahn (Subway/Train) Station, so you can get to just about anywhere directly or with only one transfer. The neighborhood is safe, and I had no issues walking back from clubs late at night. There are plenty of restaurants and bars nearby as well as a grocery store. Do highly recommend if you plan to spend a few days in the city.