It was a pleasure staying at Frank's.
The flat's location is perfect: walk one block and you'll be on Alexanderplatz. You're also next to Haeckesher Markt station (with an actual market twice a week), and several great shops including the Alexa mall. You have transportation to all the tourist spots available in the nearby stations as well.
The apartment is very well kept and has all the amenities, plus the amazing view of the TV tower!
Frank was very welcoming and so helpful! We had some trouble with trains when we arrived and he was kind enough to help and receive us at such a late hour.
If you're going to Berlin, this is a fantastic place to stay.