We had a very nice stay in Claus's flat. He and his family have a stylish, well outfitted home. And Claus was a warm, friendly host.
The location is excellent. It's close to the station that all the metro lines pass through, making it easy to get around. And it's just far enough from the Nyhavn bars and restaurants that street noise wasn't an issue.
There are 3 full flights of stairs up to the flat, but we didn't find this a problem at all, even with 3- and 5-year-old children.
Overall we really enjoyed our stay. It felt like house-sitting for a friend.