Caroline's apartment was wonderful. It's in a great location, right near the Soviet War Memorial (within walking distance of the Opera, Hofburg Palace, etc). The apartment was very clean, very large, and my wife loved it. The apartment was very quiet as both streets the building is on are residential and low traffic.

From Vienna's Westbahnhof railway station, the taxi was only about 15 Euro (for 3 people).

The apartment was also within walking distance (a bit long, but easily done on the sidewalk) to Vienna's City Airport Train, which provides a train to the airport every 16 minutes. We took advantage of this on our last day and saved some money over a taxi.