I had a great stay here. The apartment was just as described (a separate "mini apartment" at the back of Kerstin's flat, although once her partner had handed over the keys and closed the connecting door, it was totally, totally private) and nice and clean when I arrived.
I really loved the neighbourhood - the street outside had some great cafes and restaurants. One tram is at the end of the street, and there is another about a 5-10 minute walk away so whether you want to go north-south or east-west you are covered.
As others have noted, it is a 4 storey climb to the apartment, which would be challenging to those who aren't so fit. The stairs (more of a ladder?) to the loft bed are at a very steep angle: I would think anyone with even minor mobility issues would have a problem.
The apartment looks out onto a courtyard and there was some early morning general noise from neighbours, including a downstairs neighbour doing some drilling at 7.30am, which I'm sure doesn't happen all the time but was unfortunate for me!
Overall this was a great apartment for a short stay, and I'm very glad I found it. I would definitely consider staying here again and would definitely recommend it to others.