Frank's place is a great place to stay in Berlin. It's close to transport (bus, train and tram). There are a few cafes and restaurants nearby. There's also a small supermarket.
Frank was an incredibly helpful and attentive host. He provided detailed and easy to follow instructions about getting to the apartment from the airport. When my flight arrived very early, he allowed me to leave my luggage and arranged for the place to be cleaned after the previous guest. So, I was able to check in a bit earlier than the normal check in time. He even suggested a great little local cafe as a place to have breakfast.
I also really enjoyed the way Frank had decorated the apartment with lots of small touches, like an old telephone and retro furniture, as well as some very unique lighting.