Dune I can understand the praise for, but Herbert's later work in the series, not so much. I was fine with Paul's development into such a morally questionable character, which was present in Dune too, but the later works felt forced. Almost like pulp sci-fi rather than the deepness and undercurrent of darkness that was in Dune. The sequels failed for me for the same reason the movies do: Dune says everything it can, any shorter or longer and it wouldn't work. It doesn't need sequels, but it can't really be abridged either. That's just my reaction to them though, and I really can't stress enough how much I loved Dune.
I totally understand. The later books get... weird. They definitely didn't have the same depth to them that Dune did. I did enjoy the story of Warning, extreme spoiler Seriously, this ruins like 2 or 3 books. Leto and his role as self-proclaimed God-Emperor, as well as his relationship with the Duncan Idaho gholas I basically drank Dune from the original book all the way through Chapterhouse: Dune. I even went back and read the prequel books, which, while nowhere near as good as Dune, I did enjoy the backstory to why things were the way they were when Dune comes around.