Just returned from a 5-night stay at Astrid's apartment in Vienna.
The location of the apartment is excellent - within walking distance of the majority of the main 'tourist' sites/locations and close to a couple of U-bahn stops (Karlsplatz and Kettenbrückengasse are roughly equidistant) meaning it's also convenient for exploring further afield.
Communication prior to my arrival was good and check-in was quick and easy - Astrid was out (at work i think) so left the keys at a local cafe a few doors down for e to collect.
The bedroom was adequate for a single traveller with a comfortable single bed and plenty of natural light from a large fully-opening window. A potential downside for some might be the complete lack of wardrobe/cupboard space (all taken with someone else's clothes and belongings!) but if like me you're prepared to 'live out of a suitcase' this won't be a big deal.
The bathroom was equipped with shower/bath and was kept spotlessly clean throughout my stay - as was the separate toilet.
I was allowed full use of the kitchen and a small amount of fridge space. Unlike some Air B&Bs, Astrid does live on the premises so the kitchen cupboards are stocked with pots, pans etc - basically all the cooking equipment you could need which I found very useful. Whilst the kitchen could perhaps have been kept a little cleaner, it was no worse than you'd expect for such a shared resource and quite acceptable.
Finally, although I didn't see much of Astrid during my stay, when I did she was polite and I felt confident that, if I had any questions/problems etc she would be easily contactable and keen to help.
All in all, subject to the minor reservations mentioned, I'd really recommend Astrid's place as a good base for any solo visitors to the beautiful city of Vienna :-)