Katrin was easy to get in touch with and returned my call quickly when I had a problem getting into the flat, she helped me sort out the problem quickly and she has replied to all my messages very promptly.
Nice stay at Niemetzstrasse 51. Close to Sonnenallee S bahn station, about 10-15 minutes walk to nearest Lidl and some lovely little restaurants. Good location if you don't want to stay in the middle of Berlin and like to see a bit of a different side of the city.
Only downside is that the flat is right by the stairs in the main foyer so can get loud when neighbours are coming and going or talking in the foyer. Overall neighbours were quiet from 11pm onwards so it didn't really affect sleep.
For a first trip to Berlin it was a nice place to stay and Katrin is a very helpful host.