I stayed in Tanja's Kreuzberg apartment for a week and it was just right for my needs. It was neat, attractive, and well kept, and I enjoyed the location near Viktoria Park. It is reasonably close to the Yorckstraße metro stop, although it was a fair distance to haul luggage.
There was a bit of difficulty getting the key to the apartment upon my arrival, as it was not clear to me that I needed to make an appointment in advance with another person to receive the key. (That could be expressed more clearly or else an alternate plan used, for example picking up the key on the property.)
Tanja was, as promised, available via mobile phone, and she was very friendly and helpful when I had a question or problem.