Karl and Lisa are one of the most accommodating hosts I've ever met. Their apartment was clean and had everything we needed. The location is good with only a 5 minute walk from the U bahn. It is within walking distance of the Schonbrunn Palace and the zoo, it also has many nice supermarkets nearby, like the Merkur supermarket (you should try the hummus).
They kept in contact with us and made sure our stay was going well. They also let us check in early when we had an early train.
Really lovely people and I would highly recommend them to anyone travelling to Vienna.