One of the best authors currently doing his thang.
Hyperion's use of classical alliteration and its stylistic relationship to the
Canterbury Tales makes it a classic in my eyes.
The rest of Dan's books range from excellent (Fall of Hyperion and The Crook Factory) to ok (Endymion and The Hollow Man) to pure crap (Fires of Eden and Orphans of the Helix). Hey, a man has to earn a living, right?
One of his noteworthy literary attributes is that he likes to take a famous literary persona and deify it in one of his novels. The three latter Hyperion Cantos novels focus on John Keats, The Crook Factory on Ernest Hemingway, and Fires of Eden on Mark Twain. It can get a little obnoxious, but overall it's interesting.
Dan also has a way of skipping from genre to genre and succeeding in each. This is the mark of a real writer. He has gone from horror (Carrion Comfort) to sci-fi (Hyperion) to historical fiction (The Crook Factory) and will continue to bounce around in the future.
That is if the Shrike doesn't get him first.