BA gave me a free silver card a couple of years ago, so I've used their lounges at LHR and elsewhere a few times. The main advantage is having a relatively quiet space in which to sit - Terminal 5 at Heathrow is busy and quite noisy, while the lounge is busy but quiet. The food on offer is very ordinary, the wine is OK, the beer horrible, the fridges not cold enough and they keep the spirits at ambient temperature so the gin and tonic is never properly refreshing, although it's quite pleasant to get mildly tipsy at someone else's expense before squeezing myself into an over-small metal tube for a couple of hours.
If I had to pay for it myself (and I know you can't with a BA lounge) I probably wouldn't bother. I'd go and find a sit-down restaurant or pub.