Excellent experience. We were a family of three adults: father and mother aged 70, daughter aged 40. We did appreciate the presence of a lift (being at the 4th floor!). The apartment is beautiful, with a very good wifi coverage in all the rooms. and it is ideally located just in the heart of old Vienna; it is absolutely quiet and comfortable. You are within a few minutes walking distance from most of the important things to visit in Vienna: St. Stephens, Hofburg, KHM, Albertina, Sezession, Museum Qvartier, StaatsOper, Musikverein. Public transportation is available very close if you want to visit somehow more distant places like Belvedere, Schoenbrunn, Prater.
Easy checkin (we arrived indeed within the "standard" time slot; so, it was immediate to get into the apartment). We were also allowed to have a slight extension of the checkout time, and during our stay, when a couple of times we needed, we got very prompt responses when we had something to ask.