So far I've read four of his books:
Hyperion. It took me a while to get into it, and as I was reading I was thoroughly confused by a good deal of the plot and history thanks to his approach (as you probably know, seven "pilgrims" relate their backstories to each other as they journey towards the murderous Shrike, and each story is told in a different style) but overall I enjoyed it thanks to some terrific world-building and interesting characters, and that despite the confusion, much of which remained at the end, the multiple questions left unanswered, and the focus on John Keats, whose poetry is mostly too self-consciously hyper-poetical for my taste.
The Fall of Hyperion, the immediate sequel to Hyperion, about which I had conflicting views. The first half was stunning, and I could hardly put it down, since it had everything – vivid description, imaginative worldbuilding, fierce action scenes, nuanced characters who were interesting and real, genuine pathos, drama, political intrigue, thought-provoking themes and ideas, suspense, and poetry (even if it was by Keats). But dissatisfaction grew over the last quarter of the book as things were wrapped up. Sentimentality, which had been kept well at bay up until then, became rather more prominent, as did some rather contrived and unrealistic episodes which to me short-changed the whole novel, not to mention what appeared to be gaping plot-holes. I found it a disappointing ending to an otherwise very good book and it made me reluctant to carry on with the sequels.
Flashback. This is one of his more recent books, from 2011, and is a detective story set in 2036 in a disintegrating and drug-addicted USA. I found it gripping and intriguing from the get-go, but the heavy-handed far-right-wing politics (avowedly not Simmons's own, which makes the explicit and recurrent Obama-bashing in the narrative -- ie not in the mouth of the characters -- somewhat odd) became disturbing, and when looked at objectively none of the various conspiracies actually made sense. Nevertheless, a fast and enjoyable read.
Drood, which someone warned me was long and boring and I should have listened... An opium-studded tour of Dickens's last years of life, as related by his best friend and apparent worst enemy, Wilkie Collins, it's most definitely long, being just shy of 800 pages, and tedium is not helped by Collins-Simmons' fixation with telling us the minutiae of Dickens' (and his family's, friends', acquaintances', enemies') life, work, parties, interests, health, hates etc etc. I managed to get through the whole of it but only with some difficulty, though the pace did pick up a trifle towards the end. An interesting and inventive book, but even as a reflection of Wilkie Collins's vebosity and hubris as narrator and Simmons's homage to Victorian-length writing, to my mind it was far too long to sustain the conceit.
Not sure if that helps you any, save to keep you from even attempting Drood!