Frank's place was great. It was across the street from the s-train and within walking distance to a lot of the major sights in Berlin. The view of the tv tower from the windows was really cool too. We had no problems with the place and would definitely return!
The only thing I would mention is that all the dishes were pretty grimy and none of the sponges left for us were clean, so we couldn't really wash them. If you're planning on cooking a lot, this may be an issue for you.