I had a great one-week stay in Gerd's apartment and a very positive experience overall. The apartment is comfortable enough and in a really good location, off the main tourist trail but right next to Belvedere and the botanical gardens, within easy walking distance of the city centre and many cool sites. Although perhaps a little spartan, the apartment has all the important things you need, and is a great base from which to explore Vienna. Also, Gerd is a really easy-going and interesting guy, who gives you as much space as you need but is always ready to offer advice if you want it. Highly recommended, if you don't mind not having a washing machine.
Thanks a lot Gerd, and good luck!