Sanya is the Chinese Hawaii. It has some picturesque places and international hotels. If you are Chinese it is a great place.
If you come from a modern international location I cannot recommend Sanya. When there are many places in SE Asia for a quarter of the price, English speakers, international clean standards, food and water without fear of getting sick, clean crystal clear beach water, and UNESCO sites why would you go to Sanya?
150-400 US a night for a hotel can get you private villas in the Phillipines, Thailand, or private junks in Vietnam.
I'm a seasoned traveler who has toured over 40 countries outside of work and even more as an airline pilot. My advice is get more for your money with clean international standards elsewhere.