Hello Corky,
When we were on the Emerald I bought the full week pass to the Thermal suite. I really enjoyed it, but had to make a point to go, as there was just too much else to do.
As for whether or not it was worth it, since the regular sauna and steam rooms were available.....for me yes it was worth it, but only because for the week I was on the Emerald the women's steam bath was out of order most of the week. Otherwise, probably not really worth the money. Glad I tried it, probably won't do it again.