The appartment is in a central and safe location, which allows to visit easily all the areas of the city. Frank was quite flexible and made our check-in and check-out easy.
However, we were a little bit disappointed by the appartment itself. It was lacking of basic elements (no bathroom rug, no dish towel), there was no 'personal' decoration, it all looks old and some things do not work, and it did not give the impression is was very clean, which was disappointing considered the fact we pay an extra 30euros for just that.
I would not recommend this appartment for a stay in Berlin.