I loved staying at Heidi's flat. It is very close to public transportation, but I never used it as most of Vienna's sights are within walking distance. The location is in a very cool part of town with some great restaurants and bars nearby. Also: a supermarket is right around the corner to get anything you need. The apartment was furnished brilliantly with a cozy bed, wonderful shower, and wifi.
Heidi and her son were very kind and answered any questions that I had right away. I could not imagine a better place to stay in Vienna.