Wow, Dune was long, a bit complex, and good; Dune Messiah was similarly good, with a bit more complexity; Children of Dune took the complexity up a notch and really rounded out the trilogy quite well. I now feel that you have to read all three to really appreciate the first novel.
I wonder if people who write multiple part series think upfront that they are going to write a triology, or in the Harry Potter case a colleciton of seven books. It is not necessary to plan this ahead of time. If one does then it makes it easier to foreshadow across multiple novels. On the other hand, I imagine writing a single, self-contained novel is not only eaier, but also allows you some freedom to take the stor wherever it feels write to take it. The difficulty must be in the ending. There is the question of whether to ensure all threads are tied up at the end, or if it is okay to just let things remain open ended as long as there is a satisfying ending. That is easier said than done, but I think each Dune book that I've read so far has ended in a way that allows the book to stand on it's own.
The one issue I had with this book is that I think I might have to read it again at some point because there was so much that was left unexplained and meant to be understood just by context. I personally love this style of story telling and have a bit of a disdain for plot devices that are there just to explain terminology or concepts in an imaginary universe. But the flip side to that coin is the necessity to make conceptual leaps and use a lot of memory to make connections and clarifications to previous concepts as you get more information later in the story.
I highly recommend this book, however you absolute cannot read it by itself, you must read the first two books before this one. In reality, I think they are really just one long book that happens to be broken into three parts.