This tends to me among the top five books I recommend to anyone who cares to ask.
Questioning and undermining Rousseau's 'noble savage' was one of its essential goals (as Alan mentions below), hence the positioning of a classic dystopia in an idyllic setting and the choice of 'boy-scout' perfect protagonists. It is as good a dystopic novel as they come. And essential because most dystopic novels were set in urban settings, giving the illusion that extreme control leads to dystopia. Golding shows that extreme freedom can too.
It is a great work because it speaks so truly of the human tendency away from organized civilization. To me, the one fault is the ending -- the time scale given to the thought experiment was too narrow, allowing only one swing of the societal pendulum.