Paul Atreides drowned the galaxy in blood in order to slake his thirst for vengeance, set himself up as the ultimate tyrant, and became the final expression of human potential along the way. But for all his power, he died a disillusioned and embittered failure. Power robbed him of humanity, and clairvoyance robbed him of choice. Seeing no further value in life, he succumbed to traditional fremen mysticism and wandered off into the desert to die. Such is the bleak fate of heroes in Frank Herbert's universe.
Leto II's life follows a similar trajectory. Like his father, he escapes a palace bloodbath while he's still a young man, and spends his adulthood pursuing political power and the realization of his full potential. However, he goes considerably farther in both directions than his father ever did. Where Paul ruled the galaxy for a few years, Leto rules for several millenia. Where Paul knew some of the future, Leto knows almost all of it, and most of the past besides. Where Paul loses his humanity in a metaphorical sense, Leto actually ceases to be human. Where Paul despairs, Leto goes insane.
The major theme of Dune, and of God Emperor of Dune, is the folly of hero worship, which Frank Herbert seems to have regarded as a congenital defect of the human race. He grew up while the horrors of Nazi and Soviet repression were being perpetrated by quasi-pharonic regimes in Europe, so it's not hard to see where he might have gotten that idea. In God Emperor of Dune, this theme is made even more explicit than it was in Dune. Leto's "golden path" is to set up a tyranny so complete, so repressive, and so hateful, that humanity will never again fall prey to another messiah, savior, mystical hero, god-emperor, fuhrer, or whatever. Leto will grind the faces of all humanity generation after generation, century after century, millenia after millenia, until someone, somehow, finds a way to get rid of him. After that, people will finally understand. The idols must be destroyed, at whatever the cost.
Frank Herbert was an author with courage and vision, and in a way this is the most courageous and visionary book that he wrote. I think he deserves to be read for that, if nothing else. God Emperor of Dune has a genuinely thoughtful core of ideas behind the story - which isn't something you can say about every, or even most, works of science fiction. In a way, it's the most ambitious of the Dune books.
That being said, this book has serious weaknesses. His characters are not very believably drawn. That may or may not be a drawback, depending on what you look for in a work of science fiction. I don't find his characters particularly believable. Real people usually aren't dominated by a single passion, or a single idea, the way Frank Herbert's characters are. They falter, they contradict themselves, they flail around, they change their minds. By comparison, every Frank Herbert character is an intensely focused monomaniac, who plays the darwinian game of survival with gusto, and who is never weakened by a moment of hesitation or self-doubt. In a way he's not really playing fair with the audience by depicting his characters in this way. Since, as he's so keen to tell us, there are no heroes, it would have been nice to see these people falter a bit, instead of acting so... well... heroically.
Then there's this whole business of resurrecting Duncan Idaho over and over and over again, which is neither funny nor believable nor interesting. Really I have no idea why he kept doing this - it's an albatross around the neck of the entire series. Every time I opened a new Dune book, and this guy showed up for the ten thousandth time, all I could do was groan. It's the most bizarre aspect of the entire series, and it's definitely a distraction.
The world building is still there, and there's still plenty of political intrigue and scheming. But, alas, the thrill has worn off, and Frank Herbert never quite recaptures the magic of the original in this respect. It was believable in that book because the galactic civilization he was portraying was based on real midi eval models. The Navigators are stand-ins for the hanseatic league, the Fremen for the arab tribesmen who overran the middle east in the 7th century, Paul for Muhammed, the Padishah Emperor for the Byzantine emperor, etc. Conceivably he could have done the same thing with God-Emperor, going back to Pharonic Egypt or Persia under the Shahenshahs for his models. History offers plenty of examples of real God-Emperors that he could have studied, and they were constantly surrounded by conspiracies. A much better book could have been written about Leto III - the hapless wretch who has to manage Leto II's galactic empire without the benefit of his omniscience. Then we could have read about some really interesting conspiracies, as the resurgent noble houses and guilds etc. rush to fill the void left by the God-Emperor's passing.
Instead we get the conspiracy of the ten-thousand Duncan Idahos, which is both doomed and ridiculous. It has to be ridiculous, because Leto is so hugely powerful that Duncan can't possibly defeat him. You can't very well have a conspiracy against God. Or not a very effective one, at any rate. The really believable thing for Duncan to do - what all of us almost certainly would have done - is genuflect along with everyone else and then get on with our lives. Real people do that every day with regimes that they hate every bit as much, and which don't have nearly as much power as Leto's. But, as it turns out, Duncan is suicidally brave and Leto isn't exactly playing with a full deck, so Frank Herbert gets to have his cake and eat it too.
Probably the single biggest weakness of this book, though, is the way that he chose to symbolize Leto's dehumanization. Granted that it had to be symbolized somehow, the way that he chose to do it is fatal to the effect he's trying to create. Visuals matter, and instead of a visual that evokes a sense of grandeur and pity at the same time, which is clearly what Frank Herbert was going for, we get this absurd jabba the hut creature, who makes you want to laugh more than anything else. This more than anything else ruined the book for me, and I suspect for many other readers as well.
I'm not sure if I can really recommend this book. I suppose it depends on what you're looking for. If you want a book like Dune, with its weird mix of messianism, philosophy, and palace intrigue, you'll be disappointed. The elements are there, but they don't come together in the same way, and they aren't nearly as effective. However, if Dune challenged you to think about the world you live in, and you want to continue the conversation with a creative and thoughtful commentator, you won't regret reading God Emperor. Even if you end up not liking it, you probably won't forget it either.