I was asked to join them in the early days by one of the founders but I have to say I felt the service they offered was pretty basic.
All of these companies are very similar and I don't find Centurion much good either: they are good at getting things done for a price but hopeless at doing it at a competitive price. I would rather spend an hour doing it myself and save a few grand.

As an aside, the one service I find invaluable is that of my casino. Yep, I know gambling is bad and not everyone agrees, but if you are willing to drop enough dosh in the box, they will do anything (and I mean anything) for you and the service I have recevied far, far, far outstrips anything offered by any other concierge service I have used. I get collected in an Bentley limo from Heathrow, taken shopping, taken to exclusive events, complimentary Petrus...really fantastic!