Stayed in the apartment for six days and thoroughly enjoyed it. The apartment was just like listed and it provided a relaxing environment after long days of sightseeing. As everybody else already said all there is about host and their hospitality (they will go out of their way for you: we were an hour and a half late and it didn't present any problem) and willingness to provide information (Lydia even offered to drive in front of us so that we could reach P&R garage, just in case) I'll just say that i fully agree with all the good things that have been written so far and add that the bed is divine! Slept like a baby all through the week.
As I've read some reviews of the apartment's neighbourhood on the TripAdvisor that describes it as out-of-the-way and not very well connected with the center of the city I feel obligated to say that those things are only correct if you planned to sleep at the Hofburg palace itself. We were regularly travelling to the center and it never took us more than 20-25 minutes including the walk to the U-station and all. So there is no doubt - the apartment is at a great spot, just far enough from everything to provide relaxation and close enough not to discomfort you when you go out.
In a word - a great experience, no doubt about it!