This is a really great find. Fantastic location - 3-4 mins walk from the direct airport bus, right outside the U-Bahn and on the northern edge of the main historical 1st district (10 mins walk to the Stephansdom and 15 mins to the Opera House and Hofburg).
As others have mentioned the entrance to the building and the stairwell are not so attractive but the apartment itself was very clean and comfortable. Despite being right on a fairly major road in the city we were not disturbed by the noise of the traffic. There was also very little noise from the neighboring apartments many of which are actually offices and therefore unoccupied during the evenings/nights. We visited in November and we were very pleased that the apartment was nice and warm.
Sissi was a great host. The self check-in is simple with clear instructions. And Sissi herself was very responsive and helpful, answering our questions both before and during our stay.
Bottom line - a highly recommended place to stay for two people in Vienna.