Central Vienna is relatively small and you can walk to most major sights and shopping areas. Despite this, Eva and Michi's flat is situated perfectly just outside the Ring and in close proximity to the Museumquartier, Hofburg, Belvedere, Naschmarkt, Mariahilfer Strasse, Karlskirche, and Karlsplatz. The latter holds a metro station connecting all 4 lines taking you swiftly outside the city for instance to Schönbrunn or Therme Wien.
The apartment is nice and clean, and has most commodities you'll need, including a tub, a dishwasher, and a washing machine.
Eva and Michi replied quickly to all messages and sent information about the local area and Vienna in general on the day of arrival. I would most certainly recommend a stay in their apartment on your stay in Vienna.