We enjoyed our stay at Gerhard and Nicole's apartment - it's exactly as pictured, and was totally spotless. The underfloor heating is a real treat, and we found the bathroom design especially nice. The apartment is in the back building, overlooking a courtyard, which means it is very quiet. It's extremely comfortable overall, and the neighborhood is great - restaurants and little off-the-beaten track places right at your doorstep. The owners live in the front building and are easily accessible if you need anything. Our arrival times shifted, and Gerhard was gracious and easy to contact and deal with.
The only things to caution on - as previous tenants have noted, the stairs to the 3rd floor apartment are quite steep and winding and there is no elevator (not a problem for us but would have been for, say, my in-laws), and the internet was very spotty. Gerhard brought over a hotspot for us to use on day two - very responsive - which fixed the issue.