We stayed at Renate's place for a month when we relocated to Vienna. The flat is very well located, just a 5 min walk to Karlsplatz and within cooee of everything in town (i.e. Opera, Hofbug, Rauthaus, Stephansplatz, Museums, Naschmarkt, Danube canal, etc, etc)! The place is also relatively close to Hauptbahnoff and the tram is just around the corner.
The flat is located in a period building - with an old water fountain in the yard - and therefore is spacious, yet all amenities are modern and well kept. Whether you are coming for a weekend or for the long haul, Renate's place is definitely recommended.