Staying at Alexander place was a nice experience, even though we didn't actually meet him, we were welcome by a friend of his.
The apartment is clean with new furnitures in a minimalist style which makes the place spacious. It was more than enough for the 3 of us (with one kid), and would be still comfortable for a family of 4 (or even 2 couples).

It's well located, even though not too central. To access the city centre, one can take the tramway 12 which is a 5 minutes walk, the U-Bahn line 2 is also not so far so is the S-Bahn station (with a convenient direct train to the Schönefeld airport).
Prenzlauer Berg is a very nice quiet area of Berlin and there is a very nice park just a few hundred meters from the place, with big playground for kids.