Me and my friend stayed at Martin's place over the weekend. I saw Martin only once during check-in, but he was very easy-going, friendly and helpful. He responds quickly to messages and if we had've had any problems (which we didn't) or needed help, he would've been available.
The 1-room apartment with 3 beds (1 double) and a kitchenette was very tidy and clean. Everything as described - no complaints :)
The apartment is only a few minutes from Potsdamer Platz, so it has good transport connections.
Fully recommend!