This is a perfect base for a visit to Berlin. We stayed for two nights in July. The studio is clean and faithful to the description on the website. It's on the ground floor in a quiet courtyard; it has big windows and lots of light. There's no kitchen but there's a coffee machine and you can get bread and pastries from the bakery around the corner, so at least you can do breakfast.
Heidi is a great host. She lives next door, and manages to find the perfect balance between welcome, presence, and discretion.
We would thoroughly recommend Heidi's place.