I actually love the spas on the ships. As for the person who said they compare to Massage Envy, they must have a really awesome Massage Envy! The last time I went there, the lady talked to me the whole time about her mixed-faith relationship. *awkward*
I do the treatments that involve alone time, specialized wraps, and rejuvenation. It's quiet time away from the hassle and bustle of the ship and my very active young kids. I do try to book specials on board and decline all prices in advance. I think it's worth it but obviously everyone has their own thoughts!