I have yet to read Heretics or Chapterhouse but, my thoughts so far (I have read Dune many times, Messiah and Children slightly fewer and have just read God Emperor);
Dune is the bomb. It is standalone, it is so dense with language and ideas. It is the ultimate subversion of the Hero's Journey. I can't understand how anybody could describe it as a children's book! But, I couldn't understand how the 1984 movie of the same name could so misunderstand the book and render Paul Atreidis as not just the hero, but actually as a prophecised 'special one'. But, it turns out, that is not an unusual reading of the novel
Dune Messiah, it seems to me, works to clear up any doubt of that. I am puzzled by the idea that Children is somehow perceived to show some form of redemption for Paul? He has left the Golden Path for his son, knew the potential consequences of Chani's prregnancy upon Let also, having abandoned (as did Jessica) his sister to that fate. I don't see redemption.
Onto God Emperor. I honestly don't know what I think of it. I think it very much depends upon how you choose to read it, and it may well be that the clue to how it should be read is right at the end of the book. The last line that Leto speaks (whether anybody else hears it or not) is "Only fools prefer the past". Given the number of times Leto goes on his 'safaris', his disdain for the Museum Fremen (ignoring that he has created them...itself a clinging to the past, and a forlorn and broken hope that they would retain their essential Fremen-ness?) and his succumbing to his desire for human contact (Hwi) - is he saying he is the fool. There are so many ideas within the book that it becomes almost meaningless as a story. It may be, simply, the only Zensunni novel in existence? A philosophical treatise, with the added clue as to how to read those musings? The story itself is replete with instances where an utterance is made whose purpose is to begin in it's recipient a chain of thoughts, and then we read the continuing references to Leto's apologia (the Stolen Journals)
As a story it does very little for me, but as a hive of questions it does much.
Or maybe I have misunderstood. One glaring chapter really hammers home, for me, how weak the book is as a coherent story - the capture, interrogation and killing of Malky. What was that about? It served absolutely no purpose that I could see within the framework of the narrative. Like, none.
So, in short. I think the book lacks any narrative value, but I actually enjoyed reading it and will do so again.