I find it worth it if we are spending more than 3 hours knocking about the airport. Otherwise it really depends on what you do/spend while waiting to board.
I'm definitely booking one for our layover at LGW in June, with our transatlatic flight departure having changed to nearly 3 hours later and our connecting flight arriving at Gatwick now moved to 90 minutes earlier ....
I don't fancy walking around the airport for nearly 7 hours so to me it's worth booking it and in fact, adding an extra hour on as well.
It's not all about the cost. It's nice to be able to sit away from the general crowds and relax a bit. Obviously how peaceful it is depends on the lounge, season and time of day so all factors need to be considered