We loved our staying at Ulli&Gunter apartment. Not only because of the place but also because they are great hosts, always looking to make you feel comfortable.
The apartment have the perfect size and the bedroom is big!
The bed is just amazing and cozy, such a perfect night sleep!
the neighborhood and building is very calm.
To/from the city center is super easy to do by walk (20 minutes, always flat) or you have 2 trams and one subway, to choose. Also easily to/from the airport (we took CAT and then tram 2, without any more exchanges).
In between The city center and the apartment you have the Neubau neighborhood, that is amazing for a more fancy brunch or breakfast.
Vienna is an amazing city with perfect cafes and museums and opera and and and... so many things to do and see!