My limited experience on the island was really centered around the FS, although I visited The Library and a few other properties. FS has a great, terrible location.
Great in so much as it's beautifully located in a private corner of the island with rather private vistas of the sea and beyond. Terrible in that it's far from most island activities, although perhaps that's by design as staying on the property there certainly is amenable and obviously preferred by the hotel.
Overall I'd say, like I've read in other threads before, this isn't an Aman style experience, but perhaps the best the island has to offer. I'd definitely recommend an upgraded villa. I had a private pool villa, just overlooking but not directly on the beach - privacy was paramount in my selection.
Very nice villa, despite the issues with hot water and bugs, but overall quite pleasant. What wasn't quite up to par about this resort was the service. I found it to be below other FS, even those domestic properties in America (gasp!), which could have been due to language issues or simply training. FWIW, when I contacted the GM, he made it right and then some. I was impressed with the management response.
Overall if I was going back to USM, I'd stay at the FS again. But I am in no rush to return to Samui. So many other places to try, and so many desires to go back to Amanpulo.