I've read the book and it was ordinary. It is very poorly written. However, I recognise the importance of it to modern sci-fi and even some fantasy. If I'd read Dune back in the 60s or 70s it would've blown my mind (and I probably wouldn't have noticed how bad the writing was) but I grew up on Star Wars, which repeats more than a few of Herbert's ideas. I also think the 40k setting is brilliant, which is also heavily inspired by Dune.
Basically, Dune deserves credit for being the first to introduce a lot of this stuff, but if you read it now, none of the ideas are new. Plus you've already seen the movies.