I would recommend staying with Niklas's apartment if you are visiting Vienna.
First, the location is great. The apartment is located on the main shopping street. Also, the apartment is just a 1-2 minute walk to the underground which will bring you to anywhere in the city.
Niklas was a great host, he was ready to meet me for check in soon as I arrived. He responds quickly to any questions you might have. He also respects privacy and only contacted me when I had questions.
The apartment was very clean and well equiped. It has everything you need to feel at home. I will definitely recommend staying here and if I ever go back to Vienna, this will be my first choice.