Dune, the first book in the series, was not "Great" in my estimation. The 2nd & 3rd books in the series are more solidly written, and as a result, easier to get caught up in. This particular book is fairly "thick", in that it is more overtly philosophical & theological than the previous installments. This is not to be taken that it is a struggle to get through... that is, unless you want to be!
I really believe that Herbert himself found his voice in the second in the series & had cemented, by the time he penned this book, his worldview & personal religious beliefs. As a result, there is a good deal less "self-excorcism through writing" going on in this book, but a more forceful, commanding tone to it than the previous.
If a book is measured by how many perfect sentences are in it (the average book has one if you are lucky), this one is well above average. I have noted 4 or 5 truly magnificent sentences in this book (and I am only 3/4 of the way through).
His commentaries on bureaucracies & bureacrats, for example, are brilliant.
I would recommend giving this series a 600 page grace period... the payoff is huge. By the time you hit this book, you will be completely consumed.