The apartment is very spacious, designed in very good taste, and modern; makes you feel at home after a long day of travel. The apartment houses more amenities than the pictures show such as microwave, toster, hair-dryer, washing machine (which we used), fully equipped kitchen which we used heavily. In fact the pictures do not do justice with the apartment. Anyway, there is no point considering a hotel in view of this apartment.
We were only two people but seems to us that it can easily house 6-7 people especially with children.
Regarding the location. With the exception of the Schoenbrunn palace we did not have to use the U-Bahn at all. Everything is a stone throw away: a lovely market, many Sushi places (and more special Japanese restaurants for those that understand more than mere Sushi), Hofburg, many museums and the list goes on.
At the end of the day the host kept our luggage (three suitcases) for a full day.
The host provided a lot of information about the area which we actually used quite religiously. Also their recommendation for a fixed-rate airport shuttle proved to be great.
To make a long story short, everything was great start to finish.