The thermal spa is great on port days, they are open late, I believe it was 10pm on our last cruise (Jade 11/21). It's great way to recover after a busy day out and about. I have never seen the suite super busy but there is the added benefit of even fewer people on port days.
The first time I bought the passes my partner thought it was a waste of money but I have a great photo of his face in the whirlpool after an excursion that involved a good amount of walking... sheer bliss. I would post it but he would not be pleased. I'll will say since that first experience, when we book a cruise, he immediately asks if have I purchased the thermal suite passes yet.