Excellent location! The historical city center is just across the street, literally speaking, just one U-Bahn station from Stephenplatz. It's also on the inner side of the building, so it tends to be rather quite despite the location.
Great views towards the Wienerwald from the balcony, wonderful breeze, well equipped kitchen.
Emil was highly communicative, he arrived at the apartment ahead of time, and took the leisure of explaining everything about the place, as well as tips and suggestions about Vienna(You will also find a printed guide he prepared in case you forgot anything :) )
In short, great place to be for a vacation!