I think it's fairly subjective, what some are willing to pay extra for...I would say, "Go for it!" whether Aqua Class or Concierge Class (I wouldn't sail without either) but that's my opinion and I enjoy a few extras...others don't think it's worth the extra money to spend.
Don is correct, Blu is your only option (other than buffet) or specialty restaurants. Blu is only for Aqua Class cruisers, but Suite passengers are permitted two (2) nights...that's it. Concierge Class or regular stateroom passengers may not dine in Blu.
No Thallossotherapy, but there is a solarium / covered pool area so the climate is controlled, which is nice.
You can purchase Persian Garden passes on board, (Note: if You are Elite class you will receive complimentary passes on Port days anyway). You can purchase by the day or for the cruise. Generally, if you book a cruise-long Persian Garden the day you board, it's a little cheaper. If you take a tour of the facility, they may give you a discount, we generally pay $200 for a 2-week cruise (about 100 each).