Everything was perfect, and there are some good suprises in addition to what Christian wrote to describe his place. I'll let you find out !
Concerning the room and the appartment : they are clean, quiet, with some nice decoration, with all the needed equipments in the room (fridge, ustensils, bowls and glasses). And for sure nice breakfast.
Concerning the location : you can join easily each part of Berlin (die Mitte, East Side Gallery, Kreuzberg, Prenzlauer Berg) and Friedrichshain is already an attractive district (Biergarten, cinema, pub, shops,... not expensive and good). And as everywhere in Berlin, there are lots of trees, squares, interesting buildings, etc.
The best of all : Christian brings you advice in everything, so that you enjoy your stay in Berlin. You really believe that you are part of his friends !