I stayed at Petra's apartment for almost 3 months due to a business stay in Vienna. The situation of the apartment is very convenient: the subway station is just a few minutes walking distance, but if you feel like walking to the city center it takes around 30 minutes. The bed is big enough for 2 people and there is a sofa which can be transformed into a quite big bed too.
Petra was always eager to help and she was so kind to post me back some personal stuff I forgot at the apartment when I left.
I would recommend staying at Petra's apartment when visiting Vienna.