We just returned from a 3-night stay here as part of my vacation. What a blast. This is an actual royal palace that was converted into a hotel, with much of the decor and furnishings kept intact. Be sure and sneak into the grand ballrooms (which were hosting private events while we were there, but we snuck in when the events had finished and waltzed around as we hummed the Blue Danube to ourselves).
Staff is great, breakfast and the restaurant are great. This is located just outside the Ringstrasse, so it's not as close to the tourist stuff as the other 5-star offerings on/in the Ring, but in my view this is a plus, as you get a bit of space between you and the crowds. Plus, it's only 2 blocks off where the main area starts, which is nothing. Furthermore, the grounds and gardens behind the hotel are stunning, you can't believe you are in the middle of Vienna, and you are right next to the big art museum at the Belvedere with its Klimt collection. The only negative - the front entrance is a PARKING LOT. How drab. You drive up to a palace, come through the gate, and instead of wonderful gardens as there are behind, you get a PARKING LOT? Other than that, this place is a must.