Hubert's apartment is in a great location right next to an UBahn station but also within walking distance (about 30 mins) of the Reichstag and Brandenburg Gate. It is also only 5 mins walk to the Berlin Wall memorial which is well worth a visit.
The apartment is in an old block and does have some issues with hot water supply but it's brilliant location, big rooms and Hubert's helpful replies made up for this. We travelled with our young daughter and Hubert provided a cot and highchair. A friendly neighbour showed us where we could leave our pram downstairs which avoided the climb up 3 flights of stairs and made life much easier!
The Vietnamese restaurant downstairs (Lila) does great food and there is a small supermarket just 2mins walk away.
We would definitely recommend Hubert's place if you are looking for a comfortable (but not luxurious) apartment within easy reach of all of the sights of Berlin.