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Yelp
Great location, great hotel. It is on the pricey side (just paid 30 Euro for a large coffee to be brought to my room) but given it's location, the condition of the rooms (walk in closet, free standing large tub, separate shower and toilet in large bathroom) and bathrooms it makes up for that. I was also lucky enough to have a french balcony, which not many rooms have. The breakfast is among the best I have ever had in a hotel (stay in about 35 different hotels every year...). Staff is friendly, helpful, courteous and overall very professional. Concierge is knowledgeable and resourceful, valet is professional and friendly as well.
The room is extremely quiet, even though I have the window and balcony to the front (there is no actual car traffic however). The reason is that there are two heavy doors (an entrance way with an additional door after coming through the heavy and very well sealed front door) and there are also two heavy and multilayered windows that have to be opened before stepping onto the balcony or in the case of the bathroom, before accessing the outside air. If you need a quiet hotel room, this is it.
For anyone who likes to shop and wouldn't travel here for just business, the hotel is located at the beginning of the main shopping area and very close to the "Altstadt" (old town), which is known to be the largest bar in the world due to it's many consecutive bars.
The only reason why I didn't give it five stars is that the fitness room is not what I would have expected for a five star hotel.
November 2016 Update:
Went here again for business, and had the chance to visit the nearby (worldclass) art museums (K20/K21). Had a great stay again and even though the gym is important to me (as indicated above, reason why I didn't give it 5 stars at first), the consistent high performance of this hotel with every other aspect (employees, cleanliness, room service, concierge depth of service) in addition to a hotel you just don't see at any hotel in the US (really, any) caused me to give it 5 stars. I will stay here again in April and the fall of 2017 when in Duesseldorf.