Frank's place was everything I could ask for during my trip! The location was excellent; a short walk to the metro or bus stops, grocery store even closer, restaurants/bars/shops/entertainment all within walking distance. What I really liked was that all this was close, but the place is on a little side street, so it felt quiet and away from all the action when I wanted to relax. I had what I needed to stay in and cook, but I was also close enough to plenty of other places to choose from.
The place would be a little cramped if you had more than 2 people traveling, but it was more than enough space for 1 or 2. I was on my own, and I felt like it was a place I could actually live permanently, so that says a lot!
If (when) I go back to Berlin, I'm seeing if this place is available before looking for anything else.