I read the first book 4 times I think but always shied away from the rest of them and definitely avoided the ones written by his son. Have you read them all?
Interesting.
I've read the entire series, as well as the prequels written by Herbert Jr.
And I asked SF writers about this as well.
Norman Spinrad (who, by the way, did a wonderful Foreword to a French edition of the book - look for it, it's been leaked on the net) says that Herbert Sr. should not have gone beyond a trilogy... the more he wrote, the drier and stretched the prose became.
Robert J. Sawyer says that Herbert Jr., although a quite accomplished writer by himself, has made his name with novelizations more than anything else (this is frowned upon as "lazy writing" in literary circles), and the Dune prequels suffer from the same lack of breadth evident in the Star Wars prequels (it's a large galaxy, the mind cannot comprehend the sheer size of it, but somehow, Vader is the one who build C3PO, and Greedo is his childhood bully?)
While Lynch's vision did take some liberties with the original text (oversimplifications, introduction of "sound weapons"), it has remained faithful to the book's atmosphere. Also, kudos to Lynch for using as much as possible of the leftovers from the previously failed Jodorowski adaptation, primarily the H.R. Giger-designed landscapes on Giedi Prime.