My husband and adult son and daughter enjoyed this apartment very much. It is extremely big, with a casual, "funky" vibe and decor: a huge living area, with open kitchen, living room area, and dining room. The bedrooms--we used three of them--are comfortable and of good size. While there is only one bath (and we figured out how to alternate!), this is an ideal apartment for a family or good friends traveling together. (Let Werner know if you will need the fourth bedroom, though, if a friend or family member will visit for part of the time, so that he can have it prepared.) There is also a wonderful collection of art and photography books, which we enjoyed perusing.
We also really liked the neighborhood, on the edge of Kreuzberg. It isn't touristy, and yet it is in relatively short walking distance to any number of significant sights and great restaurants. (And just a few doors away is a nice cafe with a reasonably-priced, weekday breakfast buffet--terrific when you're tired of cooking.) The kitchen in the apartment is very well-equipped, and sheets, towels, etc. are supplied.
Werner is very responsive, responding to emails quickly; he met us at the apartment when we arrived and explained everything we needed to know. He also provides a useful map of the area, and he is very responsive to questions, even emailing with us while he was away from Berlin.
As other reviewers have noted, the actual entrance can be hard to find; just walk through the archway off of Axel-Springer Str., continue through the courtyard, and walk through the other archway. Then, make a right, and the entrance will be on your right! Werner will undoubtedly be there to meet you!
While it is unlikely that we all will be going to Germany together again in the near future, my husband and I will gladly rent from Werner when we return, perhaps one of the smaller apartments.