If you ever feel to read something bad, see how badly others can mangle something, read Brian Herbert's and Kevin J. Anderson's sequels.
The prequels are not that bad, if you regard them as stand-alone works, unrelated to Dune, ignoring generally bad writing. Kind of pulp scifi, i think.
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Don't read any of them. Save yourself from that.
I read fifteen pages of one of the Brian Herbert/Kevin J Anderson books and set it aside. After the quality of Chapterhouse, it was like listening to nails screeching on a blackboard. I knew Kevin J. Anderson was a hack already, but if the writing itself had been a little better I might have stuck in, just to read about the Butlerian Jihad. Nope. Garbage.
Reading Dune did make me want to re-read some of Herbert's non-Dune stuff, like the ConSentiency novels and White Plague.
But, moving on, I read This Immortal by Roger Zelazny, which received the Hugo the same year as Dune (in one of the rare Hugo ties). It was interesting, but not, I think, even close to being on the level of Dune. And, like a lot of Zelazny's stuff for me, there were sections where I was fever-gripped by the writing, followed by long passages where I wished everyone would shut up. An interesting idea -- Earth destroyed, humanity leaves, the old gods return -- but it probably suffered from coming on the heels of a masterpiece like Dune.