It was great staying in Heidi's apartment. It has a great location being only a 10-15 min walk from Potsdamer Platz, and there's a U-Bahn metro station only 5 mins away that will take you straight to Alexanderplatz. The apartment itself is very spacious and clean and has what you need. Note that there's no actual kitchen, so there's no sink - only in the bathroom - but there's a refrigerator, coffee machine and dishes to use. The photos don't do the apartment justice, it's even better in person.
The neighborhood itself is very modern and quiet, and there's a park right nearby.
The communication with Heidi was good, and even though she wasn't there for the check-in, it went smoothly.
I'd stay there again in Berlin.