I only have praise for Katharina as a host. She promptly and politely answered all of my queries before and during the trip. I absolutely recommend her as a host.
The apartment is lovely and centrally located. The building overall is very quiet. The apartment does face the main road, but the sound of the traffic was quiet white noise. Also, because it is a major road, the street is well lit, and there is plenty of activity on the street well into the evening. This noise on the street could never be heard in the apartment, apart from maybe the crowd cheering during a football match, but it was the World Cup after all. :-)
The windows all face the south, but because of the balcony, very little direct sunlight comes into the apartment in the afternoon. And that is a really good thing as it was really hot this summer. But Katharina lent me a fan.
The apartment is also part of an historic building and street, which adds to its charm. It is very easy to get from the apartment to Alexanderplatz or the Ringbahn, which pretty much gives you easy access to most of the city via public transportation.
Finally, I really liked the neighborhood. There is plenty to do, and there are many great restaurants and shops, but most importantly, it is a safe neighborhood, which is always important to me when I travel.