Very well located, within a 2' walk to Marble Arch, Hyde Park, Underground, many shopping options and restaurants.
Interesting sustainability-oriented program, where you get free breakfast for each day you forget about housekeeping (technically every other day).
Very friendly, multilingual front staff.
The room in itself is nice, but... 1) it is quite dark, as they have dark tainted split windows (which is OK if you're just sleeping in your room, but less if you have to work); 2) the windows are badly insulated (but at least you can open them), which means you have to use the heating - which is less on the sustainability side.