Concierge.com's insider take:

The expanding Mandarin Oriental Hotel Group is bringing out the best in this Edwardian pile, which now has the amenities to match its splendid location—opposite Knightsbridge Tube station with the park on one side and the favorite store of fashion obsessives, Harvey Nichols, on the other. In the glam Adam Tihany–designed bar, backlit bottles masquerade as stained glass, and the restaurant, Foliage, is one of the city's foodie hits. Downstairs, there's a big, good gym, and one of the best spas (by ESPA) in London. Decor in the 197 rooms is pitched firmly in the traditional camp, in beiges, golds, and maroons, with swagged curtains, button-back wing chairs, and a pinch of orientalism—there are lacquered and bamboo bits and pieces where the chintz should be. Unless you're a shopaholic who needs to gaze continuously on Harvey Nic's, the Hyde Park rooms are the best, with nothing but trees to look at—until the Horse Guards picturesquely parade past for their morning exercise. Service is notably attentive, with a separate guest-room manager assigned to each floor.