Amazing flat, really. There's everything you need and even more haha and the terrasse is a plus. You almost feel like at home.
It's really well located, you can get to Alexanderplatz in 20mn ish by foot. And the nearest metro is 5mn away. There are shops everywhere near the flat, with a liking for BoboQ :)
As for Frank, he's an excellent host. He replies really fast and welcomed us with kindness. He gave us all the information needed and we didn't even have to bother him because we weren't missing anything. I really recommend Frank if you want to visit Berlin :)