Quite simply, the best love story I have ever read; the best sci-fi novel I have ever read, hell - the best novel I have ever read, full stop.
Characterisation, plot, story-weaving genius, depth, breadth, epic scope, emotion, science, philosophy, it's all here. An imagination that is, frankly, astounding and a sense of the human condition that is awe inspiring.
Knocking on 40, I've read thousands of books to date. Only 2 have ever brought me to tears, and this is one* of them.
Read the Hyperion Omnibus first, to really set the scene for the climactic conclusion of this masterpiece.
Dan Simmons, I salute you.
*The other was Greg Bear's Forge Of God / Queen of Angels series.