Me and my girlfriend stayed for two months in Arnd's apartment. The location was really good. Not far from the city center, with different train and metro stations around that connect directly with the main tourist attractions. The neighborhood is really calm and more or less secure (as secure as Berlin can be). Just near the apartment there are supermarkets, a pharmacy, a 24/7....
The apartment was clean and big. Fully equipped with anything you may need to spend a long period there. And even though it is the ground floor the windows are a bit high, so your privacy is somehow kept.
Also Arnd was a good host. We only knew about him when we needed him for something.
We had a wonderful experience in Berlin.