My wife and I had a great week in David's apartment. It is really spacious and tastefully decorated. It is very well-located for the excellent Viennese public transport, with a tramline on one street and a busline on the other. You can easily walk to the Museum Quartier, and from there to Volksgarten or Mariahilferstrasse. The bedroom is at the back of the house and VERY quiet — and I say that as a light sleeper!
David and Christina were responsive and helpful at checkin. This put us immediately at ease as the place didn't have any prior reviews. The apartment doesn't have air-conditioning, but David helpfully provided a second fan which was very welcome as July temperatures were in the (occasionally high) 30s.
As is often the case with older apartment buildings in Vienna, there isn't a lift (that's elevator for you Americans :-). The apartment is on the top floor which means climbing four flights of stairs (100 steps) at least once a day. This isn't a problem if you're reasonably fit, but something to be aware of. David kindly helped with carrying our bags upstairs on arrival.
In summary: great place, great location in a great city, we would happily come back! With discounts for first listing and for a week-long hire the price was good too.