I've not stayed at Caya Espanto, but I visited the island and got the tour by the owner, Phil Gramm.
Cayo Espanto is a tiny island on the back side of Ambergris. The four villas look are very nice. One feature I like is the way parts of the walls fold out so you have in effect open walls to the water. Each unit has a little splash pool, a private dock and sitting area by the water. There's a high staff to guest ratio, and each villa has its own "butler." I'm told the food is very good, but I haven't eaten there.
It is VERY expensive. Much more than I could afford, starting at around US$600 per day, including meals but not including tax and service. These prices are really St. Barths kind of prices, not Belize prices. As to whether it would be worth it -- maybe for a honeymoon or special event vacation if money is no object. To me there are quite a few other places in Belize, both on the cayes and on the mainland, that offer a great top end experience for considerably less money, and a my own butler wouldn't mean anything to me, but it all comes down to personal choice.
Belize First Magazine http://www.turq.com/belizefirst/
-and- Fodor's Belize & Guatemala Guide