The book is like one huge monologue. It's not very entertaining. The main character mostly talks about how awesome he is and how everything he does is for the better. While this is all well and good reading the exact same sentence repeated over the course of 400 pages is not.
Another of Frank Herbert's excellent series on a planet almost totally without water, and how the ecology of the place affects life there. Might sound dull, but it isn't.
God Emperor of Dune is the fourth book in the Dune series, and my second favorite of the bunch. Only Dune itself is better.
In God Emperor we find ourselves following Leto Atreides's life something over 3000 years after Children of Dune. Yes, he lives that long and it's not horribly contrived that he does so. In fact it was setup during Children of Dune and Herbert is simply following the plan he laid out there. But ruling a multi-galactic empire for over 3000 years is not a simple thing, and Leto's got reasons for everything he does. Those reasons are impressive. I like what Herbert says about humanity here. Leto makes the ultimate sacrifice to save us from ourselves.
In addition to Leto we have a few other major characters: Moneo, Leto's majordomo; Siona, Moneo's daughter; and Duncan Idaho, the most recent in a long line of gholas with their memories restored from the first Duncan, who died in Dune.
I have to give Herbert credit. He can write with a huge scale in mind and pull it off. Sometimes I feel that Leto's words are a bit thick, but on reflection they're actually right for a character of his age and experience, and if I was a deeper reader I might not feel that way.
Another important point is that God Emperor isn't classic science fiction in the usual sense. Oh, it's a classic, but it's not a space opera full of ray guns and space battles. It's mostly conversation and description, as much of the drama is in relationships, so the interactions among characters are key.
If you liked the first three Dune books you'll probably like this one too. I know I do.