Alex and Alexandra's wonderfully located classy one-bedroom apartment is great. The supermarket at the corner was great and we found the great variety of restaurants on Bergmannstrasse excellent. The italian pizzeria near the Admiralbruche was lots of fun.
The M41 bus to Potsdamerplatz and the station very useful. We used 2hr tickets and walked rather than day tickets. You have to work hard to make the Berlin or Museum cards (with transport) worthwhile. In the end, the location in Kreuzberg is excellent...lovely quiet street.