Argh, you guys on the board here are starting to make me mad. Especially part 4 (God Emperor of Dune) ist one of the best books in the whole series. Perhaps the best. The conversations between Malki and Leto II. are awesome. Just because there's no action and laserfighting it doesn't make the book bad. On the contrary: Part 4 turns more to a philosophical outline, with lots of interesting cross-talks between the main protagonists. Man...they started talking about why Bach is superior to Beethoven or why Hitler was just a pathetic loser because "I, Leto II., killed billions, so who's the worst dictator in the fuckin' galaxy!?".
Herbert started to write more in the East European/Russian Sci-Fi tradition in part 4 (philosophical, self-reflecting) and he made a leap over 3000-4000 years to the future, which didn't appeal to lots of readers who liked the characters of the first trilogy best. But it's really worth forgetting about them for now.
The whole conversation about the Golden Path for example really makes your mind spin; after reading this part you have to put the book aside and start some thinking of your own. Granted, that book is no Nietzsche, Kant or Schopenhauer, but it's quite remarkable for a Sci-Fi book. This is the stuff - together with the fantastic story of part 1 - which you remember at the end of the day.
And the last trilogy (ok, now there are 4 books, 2 of his son's doing) is also great.
Man, I hate you guys, seriously.