I've had relative success with the spas. I've never failed to get a spa treatment at JFK, admittedly I leave on one of the >9pm departures, but there's always been treatments available on getting to the lounge 7pm-ish.

T5 in F with YouFirst is a pleasure, I've never flown ex-LHR in CW, but in the arrivals lounge, with a bit of a wait, I've always managed to get something (50/50 whether they have a treatment available, or just one of the torture chairs).

I used to have a tactic for the AF lounge at CDG T2E, as the list was left unsupervised it would be full of people's names who had departed hours ago, so I just used to hang around at the bar at that end of the lounge for no-shows. Their spa product was worth hanging around for after the refresh, before that it was like what BA's is now.