Very well situated in the heart of Mitte suburb in Berlin, close to everything, from Friedrichstrasse to the Museums Island, to the Brandenburg Gate, etc... We haven't met Jens in person but he was responsive when we needed some advice on the best way to get to his place from the Berlin airport. His rommate Fyno was also very quick at answering our SMS. Even if it is a room in an apartment , we haven't seen either Jens or Fyno following our arrival, we really felt we had the entire apartment for ourselves!
The only minor downside is that it was a bit cold in the apartment (we went in November) and I'm not sure if the heat was managed centrally or not, but we had warm blankets to compensate! Also, there is some construction work on the building across the street so it could be a bit noisy, but it wasn't too bad since the windows were closed as we were in November anyway. All in all, a very pleasant experience and I would go back tomorrow without hesitation!