This is a very nice apartment. Attention to detail is keen, from the numerous towels for us (which we appreciated), to immaculately maintained furniture and fixtures. The only think that isn't superb are the floors - if refinished, this apartment would be a real showplace. We had plenty of room to spread out, we never were awakened by other members of the family when sleeping. The multiple bathrooms were also super convenient for three of us getting ready in the morning at the same time. You are right almost in the middle of everything going on at Karlplatz, including the Opera - a 5 minute walk from the apartment (if you hit all the walk signals red!). Nearby is the Imperial Hotel which has a delightful breakfast cafe. Also the Sacher Hotel where we had superb dinner in the Rote Bar our last night in Vienna with, of course, a dessert comprised of the famous Sacher Torte! You can hop on the subway within 1-2 minutes of the apartment and easily get around the city.
Building access is easy - there is a kaypad code for the outside door and for the apartment. No keys to hassle with. The apartment is on the 4th floor, but there is a small elevator you can use to bring your luggage up. The downside for us was we (OK, maybe just me) don't tolerate heat well. In Sept when we stayed there, Vienna was unseasonably warm. Here, again, the owner had small air conditioners in each room - which saved us, esp for sleeping. Do not hesitate to stay here - you won't be disappointed!