Was introduced to it many many years ago by the wonderful Lindsey, in the Ambassador Suite. She obviously knew what she was doing and it resulted in a very powerful orgasm. And one that felt different to a "normal" one.
But, as punting mad said, I've had it done a few times since and it's just felt my arse has been assaulted. So I've not tried for ages. May give it a go again with Alice or Cara