We had a lovely stay at Justina's flat. It is very centrally located, and many of Vienna's attractions are within walking distance. A stop of the U3 line of the underground is really close as well so getting around town is super easy.
The flat itself is as described - really cosy and with a lovely terrace.
We didn't meet Justina but she provided plenty of information prior to our arrival as well as in the flat itself.
I would thoroughly recommend staying at Justina's flat.