Stayed at Kathi's place for 6 nights, can only recommend the place as it is exactly as advertised. It's a charming little flat with a nice balcony, a lot of shops and other amenities located near by, at only a few hundred meters from the metro. The location is absolutely great, in a very lively neighborhood and at 5 minutes by foot from Tempelhofer Feld, which you should absolutely visit, be it by bike, walking or even running ( it's quite a weird and fun feeling to run on the airstrip :) )
As for Kathi, nothing to say other then she was very quick to answer my messages and my various questions and the arrival procedure was as smooth as you could wish for.
All in all a very nice stay, a very professional host, would recommend it to friends without a second thought.