I was in Vienna with a friend and we stayed in Gerhard & Nicole's apartment and had a lovely time. They were very welcoming and accommodating. The apartment is very bright, clean, and hip with lots of energy-saving features. The kitchen was fully equipped and it was a great place to eat breakfast and some dinners (with lots of natural light) while we were exploring the city. It is located in a nice quiet area (which makes it great to open the wonderful ceiling windows) close to several grocery stores and many restaurants but only a ~10 minute walk to the metro, which is right at the edge of the old town center.