I stayed here a couple of weeks ago, and I had a positive experience. As a Platinum I was upgraded to what was called a Junior Suite (sort of a long room with a sofa) and generally treated pretty well. The room appeared absolutely soundproof (though faced over buildings). The two restaurants in the hotel are good, although I really didn't like the vibe of the bar. Very "Mayfair cheap" for those of you who know what that means.
I prefer the Sheraton Grand Park Lane for the staff and the faclities which is generally cheaper and in a comparable location, though for tonight the Sheraton was 100GBP more expensive, so I'm off back to the Westbury.