We had a fantastic weekend at Sebastian's apartment. He came and met us and showed us around the flat and gave us a couple of tips for restaurants in the city. It's a great location right in the heart of Kreuzberg and the flat itself is probably the nicest one I've stayed in - its really quirky, spacious and well designed...even better in the flesh!
One key point is that we had to put 150 euros down as a key deposit but this wasn't a problem as Sebastian refunded us promptly on our departure. It would be great if the flat had Bluetooth speakers too! :)
I would absolutely recommend staying here and I'd like to come back and stay here myself when I next visit Berlin. It's an absolute gem. 5 stars.