The location of Catherine's Wohnung is a winner above all. It's right next to the Janowitzbrücke stop on both the S-Bahn and the U-bahn, and not far from other modes of public transport such as trams. S7 takes you directly to Potsdam, where beautiful castles of the old Prussian rulers (esp. Frederick the Great) are a must see when visiting Berlin. All famous attractions are a short train ride away, and within walking distance there are nice churches, great markets and niche shops.
Catherine is an attentive host, very warm and responsive, easy to communicate with. Her Wohnung is sunny, clean and comfortable. A friend and I stayed there for 5 nights. Berlin is our first stop on a two week journey through Germany and Switzerland. After experiencing other places, I would say that Catherine's place feels a bit "bare", that it could be better equipped with toiletries and cooking supplies.
All in all we had a very positive experience with Catherine and her Wohnung, and highly recommend it to other travelers.