What you think of The Celestine Prophecy is likely to be determined by the category you judge it in. It's more interesting to read than most philosophy books, and it has more good ideas in it than most adventure novels. But there isn't enough plot or character in it to be a really good adventure novel, and it doesn't present its ideas as rigorously as a good philosophy book. It is one of those neither-fish-nor-fowl books, like Casteneda's Don Juan series, or Pirsig's Zen and the Art of Motorcycle Maintenance.
Probably no spiritual book in our lifetime has generated such a wide divergence of opinion. An unusual number of people have extreme opinions about the book--some positive, some negative. The book contains a great many interesting ideas, and people who have never run into them before have a tendency to think that Redfield thought of them himself--glorifying the book far beyond its worth. As we try to point out, the vast majority of these ideas have been kicking around in occult circles for at least a century.
My own take on The Celestine Prophecy is that the book is a popularization of Theosophy. In simple terms, Theosophy is what happened when the British went to India. It is a 19th century philosophy that attempts to integrate Eastern spirituality with 19th century Western physics. It is from Theosophy that the new age texts (including this one) get their ideas about "vibrations" and "energy".
Most serious occultists look down their noses at popularizations, but I don't. The major theosophical texts make hard reading, and they could use some popularization. (A. E. Powell's books are a 1920s attempt at making theosophy more accessible, but they don't go nearly far enough, in my opinion.) In Celestine, Redfield has exposed millions of people to theosophical ideas that most of them would probably never have run into. A purist will gag at the way that the ideas are watered down, but I believe that watered-down ideas are better than none at all.
That said, I wish that Celestine were a more responsible popularization. Readers are not totally to blame for assuming that Redfield thought of all this himself. The book gives you no clue that these ideas have a history, and no path back to original, more detailed sources. (Compare to, say, Zen and the Art of Motorcycle Maintenance. When Pirsig summarizes Aristotle or Hume, he tells you what he is doing. Zen can be the spark for a broad reading program in philosophy, as it was for me at age 19.)
The real test of the significance of Celestine, in my view, is what happens to the millions of people who have made it popular. They are meeting each other on the internet and in person, and are fleshing out the nine insights with their own experiences. Will they form a lasting community of interpretation that adds value to the lives of it members? Or will Celestine vanish under the wake of some future spiritual fad? It's still too soon to tell.