We had a very nice week-end in Berlin. We came for the Marathon and Heidi's apartment was ideally located, 10 mn walk to the Brandebourg gate, Check Point Charlie, the remains of the wall..
You can rent bikes at the Grimms Hôtel, 50m right when leaving the flat. We did it and it's a very pleasant way to visit the city (it's big).
Heidi was always very responsive and kind. Her husband was there to welcome us lately Friday night ; thanks :-)
The bed is very comfortable and the place is very clean.
Just note that there isn't a real kitchen, just a microwave oven and an electric kettle. OK for breakfast or to heat up a ready-prepared dish. But if you need to cook by yourself, it could be difficult.
Don't hesitate, if you're looking for a central place and plan to discover the local restaurants ; Heidi's appartement is the right place to book :-)