I found it worth popping into the first time I was on a longish connection last summer and travelling in First - the Champagne bar is quite a nice little space, and the windows overlooking the runway are good for watching big aeroplanes do their thing.

That said, it being the first few months of T5, the GF lounge staff naturally had a bit of trouble figuring out if we were entitled entry. Two of us. Both flying in First (that's why we had just walked over from the CCR). Both Gold Card holders (not that it would have mattered, but I'm just saying...). No more than a minute stumbling around with that one, though.