Heidi and her husband are wonderful hosts and were very helpful and are right next door if there's any issues. They even went out of their way to help us with an issue with the Wifi.
The location of the apartment is great. Most of the big attractions are within walking distance, and there is public transit and restaurants only a few blocks away, including a very cool brewery/restaurant.
The apartment itself is perfect if you plan on being out and about all day and just need somewhere to sleep and call home base. There's a coffee maker, mini fridge, and microwave, but that's it so it may be difficult if you're someone who likes to cook meals while traveling. The bathroom can get a little slippery as well as there's no raised edge to keep the water in the shower. We felt very safe there and the bed is nice and comfortable. If this is your first time traveling to Berlin and you want to be somewhere that is centrally located, I recommend this flat.