Stayed six weeks, wished it was longer.
The location is about perfect. Prenzlauerberg is covered in good shops, food, drinks; for unplanned days and nights, you won't need to go more than a few steps. Connecting with the rest of Berlin is a short walk to the M2, and on to Alexanderplatz. The M10 a slightly longer walk, but can be handy for getting to Hauptbonhof during rush hour, or down into Friedrichshain.
Yeah, the penthouse apartment is fantastic. After catching your breath from the flights up, the door opens into a small landing before one last flight of steps and into the hall. It's worth it. This connects the bedrooms and bathroom to—through tall double doors—the main room. The bathtub is huge. Squeegee the shower when you're done because the hard Berlin water leaves its mark quickly. Your bedroom is large; two big windows, a desk, and a pull out sofa-bed that is best slept on with feet towards the wall. The main room is a large open living room, dining room, and kitchen with a ceiling high above. Each side is lined with windows looking onto balconies. Closing the double doors to the hall isolates sound from each side of the apartment if you want to take late night calls in the main room. The kitchen has a glass top stove, convection oven, working hood, and all the pots, pans and basics covered; good meals are cooked here. Is a great space to just hang out in.
If you're lucky enough to stay with one of the hosts, Berlin will be unforgettable.