Christopher was nice, but he had another person to let us in to the apartment, a guy called Jörg. He was very unfriendly and we had kind of a bad start of our vacation.
The apartments location is perfect, very central just near alexander platz.
You should bring earplugs though, since it's placed right next to the Ubahn with trains 24/7.
Also there is kind of a weird smell in the apartment, like the ventilation system in the building is not quite working.