I really enjoyed the sequels, personally. I think the 2nd and 3rd have some low points in them with the very unappealing Alia character taking up a lot of the text. But the 4th book (God Emperor of Dune) really reframed the series as an aeon spanning meditation on civilisation and governance and institutions, while building around the fascinating concept of prescience. Then the 5th book is a real thriller with shades of the original. It also has a couple of great twists.