Spoilers
I have read from the first book Dune to the final book by Frank Herberts son, Brian Herberts' Sandworms of Dune and I have to say I've never been more aggressively disappointed with a series. When I was reccommended the book I was told to just read the first book, I didn't listen. When did a book on ecology, political struggle, assassination, and messiahs turn into erotic robot war? The only one I liked in the sequels was God Emperor of Dune because Leto II was an interesting character but it was a bit of a headache to read.
EDIT: edited the title and some spelling errors
Knowing nothing about Brian Herbet, I've been of the disposition for a while that it was in poor taste for B. Herbert to take credit for much of his late father's legacy. I was advised, by those whom first directed me towards the series, to only read the original tetralogy, so I can only comment on the first four books. Personally, I found that the first Dune book was interesting. I wouldn't necessarily describe it as heavy, but it was thorough and sometimes thought-provoking. Dune Messiah abandoned much of the political interest of the first book and seemed to be an exploration of the personal and relational development of Paul and Chani. That was a theme that was far too pronounced for the remainder of the books. God Emperor was an adequate conclusion but was not an adequate reparation for the time I'd invested into the series. Call me cynical, but I assume that the decline in quality is Brian lacking his father's ability to tell a tale, yet continuing to capitalise on the notes he'd inherited anyway.
Yeah, come to think of it I really should've stopped at God Emperor because Heretics and Chapterhouse really took a bizarre turn in the series that really felt like it belonged to a different series altogether. At the least God Emperor was still related to the first book more than just gholas.
My friend has been reading all of them recently. I myself took the advice that you didn't, I read the first book and stopped there. You've just reinforced what I thought was a good decision. Although I absolutely loved the first book, I thought it met a good ending and I felt fine with stopping there.