I think part of the issue is that the novels just aren't very good. Lots of cool ideas, certainly, but from a narrative standpoint, I think the first half (approximately) of the first novel is fantastic, but it is all downhill from there. The novel should be about Paul growing up, but it isn't: it's about adolescent wish-fulfillment from beginning to end (as suggested above), but the narrative seems to be unaware that this is the case.
Starting somewhere in the second novel, the story just totally lost me. You have these characters that come across as barely adolescent (at best) who are supposedly ruling over this immense galactic empire. All the interesting characters are gone. There is hardly anyone left who isn't a doppelganger or construct or whatever. It's all so silly, yet it's so solemn and takes itself so seriously.
There's some very vivid sci-fi/fantasy elements in the beginning. As far as an intriguing, suspenseful entry into an imaginary world, I think the early chapters of Dune are one of the few things that can be compared to Tolkien in this regard. But unlike Tolkien, the narrative falls apart as it unfolds.