I've read the reviews for Markus' apartment and I agree with all that's been said.
Yes the location is great, the apartment well furnished and clean, and Markus was kind enough, at a quite late hour, to spend 1/2 hour with us (family of 4) to detail what we should absolutely visit while in Vienna for 3 days.
In case we had missed things, there were lots of guides and brochures on hand.
Check in and check out times were also very flexible, which we highly appreciated.
What truly sets this apartment apart from others is probably that you feel the history. This is a classical building, surrounded by more classical buildings in Vienna's Innere Stadt. This is mostly what we came to see in Vienna, and we were happy we lived it all the way to our apartment.