By the time I'd finished reading Ilium (the first book of this pair) I was enthused with anticipation. How on earth, I wondered, can the author tie up all the plot threads, characters and storylines into a satisfying, coherent and meaningful conclusion. I couldn't wait to find out.
If you're wondering the same thing, I can answer you: he isn't going to.
Ilium was a fast moving, exciting Sci-fi blockbuster with a lot of good ideas. With Olympos, Simmons piles on even more plot threads and ideas until the whole thing just collapses into incoherence. With the Greek Gods, teleportation, nanotechnology, magic, alternate universes, Shakespeare, Proust, artificial intelligences, quantum effects of consciousness, shaceships, islamic fundamentalists, black-hole bombs, little green men, Mars and more, the author throws in everything you can think of - by the end I was expecting Hitler to wander into the narrative, possibly carrying the kitchen sink because they were the only things which hadn't thrown into the mix.
Long flagged plot threads are wrapped up in a couple of lines, the villain of the book just ups and leaves about a hundred pages from the end with no satisfying resolution, major characters appear and then disappear with no indication of where they have gone, and other characters have resolutions which - to put it politely - make no sense whatsoever.
Terry Pratchett can get away with using the excuse of "it's all Quantum, innit?" when excusing plot hoes in his books because he writes comedy. An author of Simmons' calibre cannot get away with it and having read Olympos from start to finish my over-riding feeling is that not only did I not understand how the book ends, I don't think the author does either. What is worse, is that there isn't much evidence of the author caring. The last hundred pages feel rushed, as if the author is as sick as you are of the whole thing and just wants to get it finished so he could go and watch Lost instead. Which is what I advise you do. It's more satisfying and it makes more sense.