I stayed here with my mum for three days (two nights). The apartment was really nice and perfectly located. Nearby there are both underground station and tram/bus stops, so from here you can go everywhere in a little time. It's also perfectly located from the airport. The communication with Clemens was really easy, he answered immediately and was always ready to help and accommodating my requests. Since we arrived earlier he let us check in earlier than expected. Everything was perfectly clean and the kitchen is also well-equipped if you are planning to cook inside. We found some chocolates in the bed and a bottle of wine on the table, so you can understand that he's a really considerate host.
I also want to point out that the neighborhood is really nice, you have a vast choice of places where to eat, also vegan-friendly. Since I'm vegetarian I was afraid that I wouldn't easily find places where to eat, instead I had many choices just near the apartment! We tried the Schilling restaurant (recommended by Clemens in the guide) and we really enjoyed there.
So, at the end I really recommend you to stay in Clemens' place if you decide to come to Vienna. You won't regret it!