I'm reading it reasonably fast, as I hope to finish it this week; I'm about 150 pages from the end of the 800-page novel. (That page count doesn't include the appendices and glossary at the back, which make it 884 pages.) I'd finally decided to reread it before Denis Villaneuve's film adaptation of part of the novel comes out. Sidebar: yes, I acknowledge the whole "this year, huh?" issue that means we don't know if it'd even be safe to release films in theaters this year. End of sidebar. Anyway. The content of the book. It's, of course, big. And it's fun to get immersed in the details, many of which got nowhere near Lynch's film version and many of which I hope Villaneuve uses. I like the array of animal life that survives on Arrakis in the novel; Lynch's film makes it seems that there's only humans and the sandworms. I especially like showing the Fremen as soldiers earlier than Lynch did; in the original book the Fremen are already fighting alongside House Atreides when House Harkonnen attacks to re-take Arrakis.
And it turned out I was wrong about something. In August 2012 I read Joan Vinge's genuinely good and compelling "Storybook of the Film" adaptation; reading the original novel, I was surprised that the Harkonnen attack really does happen while House Atreides is still moving in to its Arrakeen headquarters. I thought the swiftness of it in the storybook was due to needing to condense this huge story to the length of a storybook. I can handle being wrong.
And I hope this time, after I finish Dune, I can successfully read its first sequel Dune Messiah. I tried, in the early Nineties, but drifted away from the book and didn't finish it.