I liked some of the ones after the second more than the second. But I honestly wish I'd stopped at the first. It is such an amazing story that gives life to an incredibly interesting and well built universe. Then, having built that wonderful universe, the follow-on books got bored with it and decided to focus on ... fancy sounding bullshit, for lack of any better description. Going back to the first book, I see that the flaws were there already, but they were in service of building the world, while that relationship flipped afterward.

This. Dune is a self-contained masterwork. The rest... not so much.

I agree that books 3-4 are pretty slow and uninteresting, but one of the major aspects of Dune is the rise and fall of tyrants, and our obsession for saviours, which only really shines in the Heretics/Chapterhouse books, where the empire really falls.

It's difficult not to see modern politics in that light afterwards, with its cult around personalities rather than ideas.

Messiah and Children are sometimes published in one volume since Messiah's so short, so you may as well read both if you're going past the first book. And if you've done that you may as well finish the arc with God Emperor.

Reading past that is, truly, not worth it. Story's over, stop reading.