In reply to John_Hat:
The first three were pretty much a single story though - a long story spread across a trilogy, like The Lord of the Rings but published separately. "Dune" does not end properly, and "Dune Messiah" certainly doesn't. "Children of Dune" ends that phase.
I never got past that, because book 4 was so far removed. Many say the second trilogy was actually good though.
But I think you are being dismissive of books 2 and 3. I have not read them since 20 years ago but I recall great things about Paul as the unnamed blind messiah, the assassin Scytale, the cult of Alia, the children Leto and Ghanima, and the Ghola resurrection(s) of Duncan Idaho. I thought all that was good writing.
btw if you like the heavy detail that Frank Herbert puts into his writing, and fancy a belting good read, try "The White Plague".