I've only been to Yap...and it was awesome! Apparently, it was a bad time of the year (october) because the mantas were switching from one side of the island to the other. But, we saw mantas on every manta dive, including one that actually "flew" over my head and touched me gently with her wing. I could have reached up and touched her, but I was frozen at that point. It was incredible.
Besides the mantas, the fish and coral life is SO beautiful! These corals are untouched, and I saw more baby sharks in the channels then I could count.
Best thing about Yap (no, I'm not a travel agent..I just LOVE the island) is the culture. It is so untouched by western society, and it is their choice. They, somehow, managed to keep their culture in tack in spite of everything. The stone money, which is still used to buy land and seal agreements, is centuries old. There isn't alot to do topside, except watch the sunset, drink straight from coconuts, and forget everything...to me, its magically and definitely worth the trip.