Heidi is an extremely accommodating host, and I'd recommend her place for a straightforward, great value stay in Berlin. She let me check in early since I had a red-eye flight, and she was consistently warm and helpful through the visit. We mostly spoke in German--which is nice if you're eager to practice, since most people in the city speak English--but I'm sure you'd have little trouble communicating with her in English.
As many of the reviews mention, the location is safe and good in general -- super close to buses, S-bahn and U-bahn, and tons of good (and ridiculously inexpensive) food and bars in the area. It is a bit of a trip to most museums and things, but I find everything in Berlin pretty far apart, and public transport is very easy and inexpensive. The apartment is also close to the canal, which makes for a pretty walk.
Some things to know about the apartment: There's no dining table, so while there is a kitchen you can use, there isn't a ton of space to eat. It can also be a little stuffy in the apartment, which I suspect is worse in the winter, and there's a lingering cigarette smell. But it's easy to air out the room you stay in -- there's a big window that opens and closes easily. The bed is also rather low, which was not a problem for me but is worth knowing for those with mobility issues (the apartment is also a walk-up). But the bedroom you will have is a good size and there are shelves you can use, as well as a chair and side tables.
Overall I was extremely happy with my stay and grateful to have such an attentive and helpful host, as well as a private room at a super cheap rate!