The apartment is great overall. The location is perfect, along Mariahilfestrasse, the Oxford Street of Vienna, yet it's set far enough back from the road that there isn't any noise from the road. It's about a 15 minute walk to the Ringstrasse and the Natural History and Art History Museums.
Another reviewer mentioned "several flights of steps" - I counted one flight of about ten steps to get to an elevator, but then the elevator took you the remaining five floors. There are single steps around the apartment so it wouldn't be suitable for someone in a wheelchair, but steps weren't an issue for us. Obviously there's a flight of stairs to the roof terrace.
Checkin was easy - the instructions were very clear and hassle-free.
The apartment is huge, with big bedrooms with lots of wardrobe space and big comfy beds. The living room is also huge with a large sofa, armchairs and TV with Internet TV (Netflix/Youtube).
The kitchen is pretty small with a microwave, electric stove, oven, toaster, Nespresso coffee machine, kettle and dishwasher. There's a big dining table. The living room windows now have curtains so the privacy issue mentioned by a previous reviewer was fixed.
The bathroom had a shower cubicle which worked well, and a washing machine. We found that washed clothes dried quickly up on the roof terrace.
The roof terrace was a nice addition, with nice views and a nice place to sit. Note that there are now two dining tables with chairs instead of the deckchairs.
Only small criticisms - a few consumables like dishwasher tablets, laundry liquid, bin liners would have been handy and saved purchasing big boxes.
All communication was speedy (within minutes) and helpful, and the apartment was very clean.
We would love to stay again one day, we recommend the apartment.