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.