The apartment was spotless and equipped with everything that a traveller might need. The location is very good - quiet and in the meantime as close to the center that it was cheaper for the four of us to ride taxis rather than metro.
Clemens was very hospitable and attentive, whenever I had any quiery about Vienna, he was quick to help with answers and suggestions.
Overall, a good deal of the nice impression from that visit was due to this very lucky choice of the apartment and the host. I strongly recommend this place.