Spent a month in this apartment. Christian was mostly absent from the apartment. But also quick to respond to any inquiries through WhatsApp.
I got pleasantly surprised with the apartment. I especially liked the cleaning lady that came weekly.
Location of this apartment is also awesome for someone who is first time in Berlin. Lot of landmarks and museums are close by.
On the flip side, the apartment is next tu S bahn rails so you have to close the windows before sleeping. After closing the windows the noise of S-Bahn wasn't a problem, but in August it was maybe hot a bit.
To conclude, this apartment gets a big RECOMMEND from me. Hard to fault it anywhere. Whoever books this apartment, if they behave respectful, won't be disappointed.