I have used it several times, most recently in August. As a PM (ST Elite +) on a Y ticket, I was welcomed in. It is on the 2nd floor of the Soffitel (formerly Le Meridian) and as ClipperDelta described, it is sort of a suite of hotel rooms attached together.
I should also note that the host/attendant that has been there every time I have used it is exceptionally gracious and hospitable. When I once had a lost baggage issue and needed to contact AmEx to make a claim, she called around to find the AmEx number for me. By the time we got that taken care of, I had to run, literally, so I could get to the GEX and then to Euston in time for my train. While I hurriedly used the facilities, she packed me a little breakfast to go. When I came back a few months later, she remembered me and asked if I had made my train. (I had). ^ ^