We stayed in Christian & Anna's apartment for a week.
The location is great - right on the edge of the Mitte bordering Prezlauer Berg. There are nice cafes and small restaurants in close proximity and you're in walking distance from some of the nicest/hippest/most current areas in Berlin. If you have kids then there's a big park with a great playground right out the door. Also, the location is quite near to a large U-Bahn station.
The apartment itself is quite big. All the rooms are spacious and there's a separate dining room and living room. It's also nicely decorated, although the abundance of items leaves little place for your own things. The kitchen however is quite small (although adequately equipped) and the bathroom/toilet combo is tiny for our standards. I understand it's common in Berlin, but for us it wasn't very convenient.
Christian was a phone call/text message away for assistance and was very helpful with any issue we called about. He provided us information on the area and good recommendations, and was generally a very nice host.