I absolutely love this series... It may sound strange, but these books continue to haunt me well after I finished reading them. My favourite is 'Fall of Hyperion' - the narrative viewpoint is brilliant, and I just love the concept of a Keats-cybrid. Simmons manages to weave questions of theology, philosophy, literature and history together in a way that astounds me. I've even managed to convince a few non-scifi readers to pick it up, and while they struggled at first, they too were amazed by Simmons's brilliant writing and unique style. I haven't read any of his horror, but plan to get my hands on some when I get some free time!
Really, I can't speak highly enough of the Hyperion cantos. While I prefer the first two books, the Endymion books are great also. My only issue is that, particularly in 'Endymion', Simmons seems to be stringing the story out to fill pages - some of the worlds through which the characters travel seem a little unnecessary. I know that Simmons likes to create unique places, etc., but did anyone else feel that at times it just went on too long, and didn't really serve the purpose of the novels?