We had a very nice stay in the apartment in Nord Mitte. It's not the most modern apartment, but the apartment was clean and tidy, and the beds were super comfortable. There is a lift in the building, but it's only 1.5 short flights of stairs up.
Checking in was a breeze with the instructions provided. It was a relief that we didn't have to worry about coordinating times to meet someone in person as the keys were made available in a locked box.
We appreciated the guide books and brochures made available in the house which helped us find a great Italian restaurant nearby when we arrived at the apartment late at night.
The location is great. You're close to the Bernauer Mauer (Wall) memorial, close to an s-bahn which goes straight to Brandenburger Tor, and you're in easy walking distance to various shops and cafes. We took the #12 tram (runs directly in front of the apartment) to the Prenzlauer Berg area for dinner one night which was great.
Also it was nice that we were allowed to check out so late as our flight was at 8pm.