I highly recommend Ulrich's place.
The location is perfect- very central, I walked everywhere, yet very quiet. I liked being on the third floor, there was a good breeze, and lots of light. The flat is huge, could easily accommodate 6 people. There is a very nice coffee place at the end of the street, and a supermarket. There are also many restaurants around (I recommend Sluka, next to the city hall, they have amazing cakes).
Ulrich was very responsive to my questions, and provided lot's of information. Will definitely come back when next in Vienna. Thanks Ulrich!