I have used it lots of times, but not for at least 5 years. There are a few of them, the one we used to use was between terminal one and terminal two, andit was Landside. There is also one airside, and I think one in terminal 4.
It's a nice comfy lounge, they have drinks and food available. i seem to think soft drinks were free, but you had to pay for food and alcohol. We mostly used it for business meetings (I worked at BA and so we spent a lot of time hanging around the airport waiting to fly home). But they had pcs you could use, and I think they had showers but I can't remember.
I have used another lounge at Manchester and it was nicer than the Heathrow one, you got free food at that one, but the Heathrow one was ok. If you have a Diners Club card then you can go in for free.
Bev