The apartment is very nice, clean, equipped with almost everything you need, and the location is excellent. We only used the metro to go to Schönbrunn, and walked to the city center every time, since it was a really nice walk.
The only downside (for us) was the late check-in. We had an early morning flight, arrived in Vienna at 8am and had to wait with two small kids around the town for many many hours.
Other than that, everything was very nice.