Frank was very prompt in replying about the booking, and provided a lot of good information about getting to the apartment, the whole set up etc.
The apartment was clean and well-ordered when we got there, and everything was as advertised - though the WiFi was a bit slow to start with, this got resolved.
By the way, it's in a great location - easy walking distance to the beautiful Gendarmenmarkt, walkable to all the central tourist sites (Museum Isle, Alte Berlin, Brandenburger Tor, Holocaust Memorial, Reichstag), very close to the U-Bahn and S-Bahn, just around the corner from Checkpoint Charlie... not bad really!
Frank came to meet us on the Saturday morning, just to check that everything was fine - and he gave us a really good recommendation about the Saturday market in Kollwitzplatz, which ended up making our day.
Overall, a good experience that suited mine and my friend's first time in Berlin.

Cheers!