I just finished the book yesterday. I rank it #1 on my all-time SciFi list. I would say A Fire Upon the Deep is #2 and Dune #3.
After a week of reflection, I think it was a slight overreaction -- the book is still very very good but i dont think #1 for me.
But something about the way the story took shape and culminated in the final third of the book was so engaging, and so satisfying. I think I stayed up until 4 or 5AM the night I finished the last 200 pages.
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Tangent: Have you read Gene Wolfe's Book of the New Sun? It's less famous than the books above, but (IMO) it's in the same rarefied company as Dune and J.R.R. Tolkien's work.
I haven't even heard of Wolfe's book. Will check it out, thanks.