Sonja’s place is a good choice if you plan on visiting Vienna for a couple of days. It’s a bit far from the city center if you want to go by foot, but the subway station is 10 minutes away, so we didn’t see this as a problem. Mariahilferstrasse is also very close, so there are lots of shops and places to eat, as well as streets to wander on. Supermarkets and bakeries are near. Schonnbrun palace is also pretty close, we chose to walk and it took around 25 minutes to get there.
Sonja’s husband greeted us and he was very polite and friendly. As other comments mentioned, the building is a bit scary at night, but if you have a light on your phone it’s ok. The place itself is clean and equipped with everything you need for a short stay. My only suggestion is maybe adding a fan to the apartment because in the summer it can get pretty hot.