Location location location! Stefan's place is in the middle of the action, just steps away from the opera and easy access to the Metro. If you want to be close to all the tourist sights, shopping, etc. this is the place to be.
The flat was just like the pictures, with a sunny courtyard view and modern furnishings. Clean and comfortable. Similar to a hotel, be sure to bring your own toiletries as basics were provided (towels, hair dryer, etc.) I appreciated the flexibility with check-in as well when my train arrived late.
While a central home after a long day of exploring was nice, I was disappointed to discover the noisy neighbor upstairs. S/he comes home at 1AM on weeknights and was up again at 6:30 with ongoing noise from scooting a chair around on hardwood. (This may have been a small child on a scooter toy, perhaps.) I'm used to urban noise, but the chair was jarring enough to wake me. Worth noting if you're a light sleeper.
Regardless, this is a prime spot for visiting Vienna; you won't get much closer to the opera than this!