We stayed in Wolfgang's flat for 4 nights and it was a great stay. Check in was easy via a video sent a few days before arrival and Wolfgang also allowed us to check in a little early so we could explore the city in the afternoon without our bags. The flat was bigger than we expected in the living area, the bedroom was as it looks in the photos provided. The bathroom was nice with plenty of hot water and a powerful shower. Unfortunately, all of the windows in the flat look out onto internal courtyards so do not expect much light, though we didn't mind as we were out seeing the city not sitting in the flat!
It is well linked with 2 U-bahn stations about 10 mins walk away (U4 and U6) and also by tram. Walking to town took us about 30 mins to get to Stephansdom. There is Spar and Billa supermarkets 2 mins walk away which were handy for picking up sandwiches before heading into the city.
Overall, a great stay in a well positioned flat! Thanks again :)
(On a side note, we found the ViennaPass card a very worthwhile purchase for site-seeing in the city).