On the Spirit right now. Purchased the spa pass for $259 for our 11-day Alaska cruise. Feel it's been a waste of money. On Day 8 now and only used the thermal suite twice so far. It doesn't have a pool. Doesn't have a hot tubs. The thermal loungers don't get very hot (very, very low heat in one specific spot, the rest of the chair is ice cold). Ambient air temperature is very cold. The lights are often turned up very bright and you have to wear a hat or hold your hand in front of your face if you want to read your phone when in the heated loungers--although you can ask the spa desk to turn down the brightness. They will do it, but it also seems to turn down the light level in the spa lobby, so they often turn it back up to full brightness in the mornings. They do have a co-ed steam room and sauna -- I have not personally used, but my wife has and she said those are fine.
I have used the thermal suite in Epic (long time ago), Encore (last year) as well as Star (this year)--all were significantly better. The thermal suite in the Spirit should just be demolished. I really regret the purchase of the pass. (No issues with Epic, Encore, or Star--those we're all worth the money.)
Maybe they will get the thermal suite on the Spirit fixed one of these days. I did see maintenance crews working on the loungers for many hours one day trying to increase the heat, but it doesn't seem they were successful.