We had a great time at Martin's place. Great location, on a quiet street, but only 5 min from Rosa-Luxemburg-Platz (U-bahn and Tram stop). One stop on U-bahn to Alexanderplatz, and a couple of direct trams to Berlin Hbf.

The flat was lovely, clean and spacious. The main bedroom is down some steepish stairs, so perhaps not for little children or those with mobility issues. Beds were comfortable. Kitchen was simple but functional. Bathroom was spacious, clean and warm. The shower was awesome!! - one of the best I've experienced! High pressure, full range of temperatures, and very responsive temperature control. It heated up to desired temperature almost immediately.
But the best feature was Martin himself. He was extremely responsive, friendly and communicative. It was rare not to get an immediate and helpful response to a message. He provided a very clear video explaining how to get in, and was happy to allow us to leave out bags before check-in. I'd stay here again anytime!