Just finished a stay at the W Leicester Square last week and had a wonderful stay.
Used SNAs to confirm a Marvelous Suite ahead of time. Room wasn't ready when I arrived at the hotel, before check-in time - was offered a courtesy room for my use as my room wasn't expected to be ready until later in the evening. I was not expecting that and was fully ready to use the spa/gym showers for a quick refresh and to head out into the city until my room was ready. Major props to them for offering this.
Only complaint about the room I have is the lack of a mirror close to an electrical outlet. I think someone else mentioned that London/UK building codes prohibit electrical sockets in the bathroom/sink area, as there are no electrical sockets at all in the wet room of the Marvelous Suite. However, there are no mirrors in the main room, where there are electrical sockets. Really hard to blow dry and style your hair without a mirror.
Also a bit noisy from the noise outside as sound proofing was a bit lacking, but the noise tended to die down by 10pm-11pm so wasn't a big issue.
Overall a wonderful stay at a great hotel in a great location!