FWIW, I just stayed at the Marriott Grosvenor Square location and it was…very average?

Check-in staff was not particularly friendly, room was ok, but a bit dated and incredibly warm with the cooling not doing much (they couldn’t process an upgrade as a gold, which was fine as they were fully booked and I don’t expect upgrades much). Location is fantastic (which was the purpose for me), but if you’re going to spend 650/night (it was a 50k point redemption for me) - I’d probably look elsewhere in London unless you absolutely have to be in Mayfair.