Martin's place is indeed the place to be. It looked a little way out on the map, but it's actually in a really good spot - it's easy to get to the U-Bahn and only a few stops from key sites like Check Point Charlie and the Brandenburg Gate. Martin was incredibly welcoming and helpful, meeting us in person and keeping our luggage safe after we checked out. He let us know about the local highlights and what was happening while we were there. The apartment was very comfortable with everything that was needed and more. The layout is a little unusual in that the bathroom is accessed via the main bedroom so it's recommended for people who know each other well. I definitely recommend this place though.