Just as indicated in the public info, this is a small ground-level studio in a gorgeous building. It used to lodge the concierge, and it looks onto the backyard—no privacy, safety, or noise issues, however. The street is very quiet, tree-lined, and some 2 minutes away from an accesible subway stop, as well as many amenities.
From Südstern it is very easy to access hip Neukölln, or any section of Kreuzberg. Mitte/Alex is not too far away either, and access to Schöneberg/City West only requires a simple subway connection. Summing up, very good location for travelers who are looking for more than just the two or three tourist sites.
As for the apartment itself, it seems to have been renovated recently: while it is somewhat basic, everything works well, and it has pretty much everything you need (personally, I was glad to find not one, but two coffeemakers :) ). It is also incredibly quiet.
Last but not least, Betty and Stefan were very accomodating and prompt in communication.
I would recommend this option specially for travelers who want to enjoy the less touristy side of Berlin—although the area around Südstern is definitely gentrified, you are not likely to find hordes or visitors.