I have just spent 5 days in Berlin with a friend of mine. I highly recommend Martin's place. Martin is great, he's a very helpful and reliable host. He also gave us a few good hints on places to go for a good typical German dinner or a fancy drink. The apartment where we stayed is walking distance from Potsdamer Platz, Checkpoint Charlie and Tiergarten. It's also very close to different U-Bahn and train stations. The apartment is very new and clean and the neighbourhood is very quiet and safe.