Myself and two friends stayed in Sonja's apartment for 3 nights, and it was perfect! The apartment was nice, clean and had everything we needed! The location was great - very easy to get around and very close to multiple metro entrances, bus stops, as well as the train. We walked to all of the main sights (Brandenburg Gate, Holocaust Memorial, Topography of Terrors, TV Tower, and more). There were also loads of great restaurants in the area - overall an excellent place to base yourself.