My family visited me and I booked Felix' apartment for their stay. The apartment was fantastic! Much better than you can see on the pictures. It is really big and bright and we felt really comfortable! Everything is really clean and tastefully decorated. The location is perfect for sightseeing, as you can walk to major tourist destinations, restaurants, cafés, bars etc. It is also really fast and easy to reach it from the airport (direct airport shuttle just 200m away)!
Felix and his flatmate were really nice as well. We had the chance of meeting them and they recommend us lots great of places to visit and to for food/drinks! They are really lovely people!
I would definitively recommend the apartment to everyone who visit Vienna!