You can't deny the sheer scope of achievement that Dune represents. The amount of thought and careful imagination that must have went into this novel and its five sequels is frankly astounding. For that alone I should probably give it five stars.
However, the way Herbert chooses to tell the actual story of Paul Atreides (showing us small windows of time that span years of events), while leaving me in awe of his creation, also prevented me from becoming emotionally attached to it any way. I liked reading about these characters and their world, but I didn't care about them the way I wanted to. Honestly, that may just be personal preference. I'm an emotional reader. It's far more important to me that a book be true and resonant emotionally than intellectually.
So with that in mind, four stars. I should probably also note that I liked this book enough to read all the sequels, even the ones by Kevin J. Anderson, professional fanfic writer.