The apartment is really far away from the center of berlin, the problem is partly resolved by the efficency of the public transport, but you still have to consider at least 30 min to reach every main attraction.
The block is really sad, so go around in the evening it isn't an idea that I advice so comes back the fist problem about the distance to the center of the city.
The apartment it self is nice, really tidy and clean and got everything you need to make confy your stay. But you shlould consider that it is really tiny (on the second floor a person 1.60 m high has to be hunched and also the bathroom fixture are a little small).
The assistance is really good, they will answer always in less than an hour and are always really kind and exhaustive.