My DP pays about £100 for himself, platinum membership including rackets and towels, then I'm about £50 and our DCs are £33 each. 
Its bloody extortionate and I certainly couldn't justify it just for myself, but he has a friend who goes so they play tennis and go to the gym together, which helps to motivate him to get up at 6am - if he went on his own I think he would bail a lot more often!
Its great to be able to play badminton, swim, use the gym etc with the kids - its got us all up and about in the evenings/weekends and its given me a place to relax, as they have a lovely spa area with hot tub, steam room, sauna etc and a really nice cafe. I always choose hotels with a spa/pool so its like being on a spa break permanently!
I didn't think I'd use it much, but I go most days at the moment and the outside pool has been amazing in this weather. Its like being on holiday all year round, and given how much that costs, its actually good value!
And no, I don't work for them, just a happy and enthusiastic new member who fortunately doesn't have to pay for her own membership 