I stayed at Alex's apartment during my work trip, located in a pretty quiet nice region nearby KaDeWe and Wittenbergplatz metro station - not more than 5 min on foot. The apartment is spacious, clean and fully equipped.
Alex is a very good host, very hospitable and friendly, he provided us with everything we need include city, metro and bucycle's rental maps. I would stay there once again especially if I go to Berlin with my collegues, who could stay in the another apartment located in the same building which is very convenient for the business trip. Thank you Alex!