Dune is not a well written book. It falls somewhere between trashy YA and whatever the literary equivalent of a reality TV show. Its pacing is erratic, its storytelling jarring and the plot... well let's just say Mr Herbert must have been on something spice like as he was writing it. The mental burden it places on the reader goes beyond reasonable and that it's considered a classic was a puzzle I constantly encountered throughout my time reading the book.
By the end I understood. Despite its flaws Dune presents a world (or rather universe) that goes far beyond what's written on the page. It deals with, or rather touches upon, subtle yet broad themes. I realised that the book wasn't punishing the reader, but was far more ambitious than a book should be. That was both its failing and, more importantly, its success.
I suppose another reason I didn't immediately champion the book was due to the lack of care given to its characters. Dune is a book where the world takes centre stage and not the people, and that showed throughout. Ironically it was the science, technology, sociology, politics and culture that came to life at the expense of the humans in the book.
In many ways it draws a parallel with what I do consider a sci-fi classic: Asimov's Foundation series. That also, by its nature, had very flat one dimensional characters. And yet I don't remember struggling as much in that case - perhaps then the key here really is design and pacing, with Dune feeling much more than a single volume.
Ultimately I've decided to continue with the series, at least till I complete the essential trilogy. I suspect Dune is a series that only gets better as you experience more of its world.