We stayed at the Adagio in November for four nights and had a great experience. It was our second visit to this hotel, after a nice week a year ago.
The hotel is located in Berlin west, one block from Kurfurstendamm, and close to the U-Bahn. Berlin is huge, and the hotel looks far from the eastern sightseeing spots, but I guarantee that it's perfectly located for people willing to use public transportation.
The rooms are actually studio apartments, clean and comfortable, with all the amenities needed. The staff was cordial and helpful. When you stay more than three nights, rates become really interesting. I would stay there again with no hesitation.