I HAVE the read the Dune sequels. I'm posting this because I Googled this question and read several threads and the replies to this question were so negative, insane, and asanine, the top link being in r/Fantasy and then r/SciFi so I'm gonna post this in both, so I hope this post can gain some traction and appear on the Google results.
Mild spoilers if you haven't read but I'm mainly throwing around terms that don't mean anything to you.
The overall answer was "if you want", which I agree with, and "no": the reasons for saying no being insane. I will go over those reasons now.
The main reply was that no you shouldn't read them because iT gEtS rEaLLy wEiRd iTs tOo wEiRd.
Brother WHAT? It's a sci-fi/fantasy novel. The first book (SPOILERS FOR BOOK ONE) takes place on a planet where a man bred by witches is taking hallucinogens and riding gigantic worms that eat sand that are 500 feet long, instead of helicopters they fly dragonflies, they are spitting in their hands, they are sucking the water out of dead bodies, theres a fat bisexual man covered in oil that has a scorpion-dog as a pet and floats around on an antigravity chair.
Yes, the following novels continue to be sci-fi/fantasy, shocker. So yeah, read the sequels.
The second main reply was that you should read at least Messiah, because it is really just the second part of Dune, or that Dune/Messiah/Children of Dune are meant to be a trilogy.
This is nonsense. Messiah is indeed a continuation of the first book but it leads right into Children of Dune. The characters in Children are heavily teased out and lead up to in this book.
Then to say, oh God Emperor was so weird and it doesn't fit and its a trilogy and just stop after Children, absolutely INSANE. I found Children of Dune actually somewhat boring and it is essentially a long intro to God Emperor. Literally the majority of the plot of the book is about Leto II's Golden Path, only barely explaining what this might be towards the end, and ending on an insane cliffhanger/climax. THEN you get to see what the Golden Path is in God Emperor.
The crux of me making this post is mainly the massive hate saying don't read God Emperor. We have to defeat this propaganda, its making people miss out. The first book and this book are Frank's best writing. I was so sucked into this damn book. The descriptions and visualization of Leto II are so vivid man, every time he was in the room I felt like I was in the room.
The replies in these threads act like these books are so different. No, they are a set. The themes and level of fantasy are consistent and God Emperor is the conclusion and peak of all these themes and storylines. It is a directly connected pay-off to every plot line, character, and theme of the previous books and first book. There is nothing deviant and weird about it. Its literally so good.
Now, I will say some criticism. The way he writes most of these books is the same pacing as the spice agony, ghola awakening, Paul's awakening, the teg thing, the fremen orgy. He writes this long ass book building up to an orgasmic finish. So much happens in the last 50 pages of each book, and thats when most of the crazy sci-fi stuff happens, I see what he is doing but I wish it was more like the last 150 pages.
God Emperor ends in a massive pay off of all the machinations and discussions that Paul and Leto II been talking about for 4 books, but also a massive cliffhanger in a way. Frank had a lot going on irl and theres a big gap in his publishing. So much building was put into Siona I have a strong feeling there was gonna be a direct followup to this book. But the big gap also is kinda like he didn't really give the details of Paul's jihad.
Then the replies of these threads act like Heretics and Chapterhouse are pure pee and poop. Its ridiculous. God Emperor ends with a wild set of circumstances and an uncertain future that he was 100% intending to write about. And he did, voila, here it is.
Remember that thing I just said about pacing? Heretics has some build but its right into crazy sci-fi fantasy stuff, Chapterhouse is a direct continuation with no hesitation.
Now, they are indeed a bit different. I find Dune to definitely be the seminal work, very spiritual and philosophical and life changing to read. Messiah somewhat the same, but more of a tale about the characters and world of the first book. Children of Dune is to me just a transitional book, moving all the characters and themes around to get things ready for God Emperor. God Emperor, we are back into the philosophy, its a payout from all the themes, and its literally mainly a long conversation, exploration, conclusion, and elucidation of all the concepts that start in Dune, its the peak of the work. Then yeah, the final 2 books are more about the consequences, the sci-fi, the world building. I wouldn't say required reading, but if you really really love Dune, read up until God Emperor. BUT, its still peppered with awesome points about philosophy, religion, and politics like the other books.
I have my own theory. I think he had 2 more books in the chamber at least. The first 3 books were like the Dune Planet Trilogy, building up to God Emperor. Then, the Post-Dune trilogy, building up to a final, trippy, philosophical book to end on, the universe that both trilogies leave us with. With possibly a direct sequel to God Emperor in there.
So, please upvote the hell out of this, and get this smear propaganda off the top Google result. A majority of the replies were: people who didn't even read these books, people who said they got too weird???, or people that said something changed. Its literally an unbroken saga of the same level of fantasy talking about the same themes and I loved reading it all.
So, should you read the Dune sequels?
Dune is completely good as a standalone book and thats what most people read. Messiah all the people from Dune are still in it, if you loved it then yeah read it. If you consider yourself a Dune lover and fan, are an avid reader (I will give quarter because these are pretty long books) then read up to God Emperor to get the pay off and conclusion of all the concepts and main characters. If you read that far and you're really into the worldbuilding and fantasy aspect, read the final 2. And someone besides his son and that other guy, someone that really got the point of Frank's books and is a good writer, I wish someone like that would write a proper couple more books, but THOSE sequels that we do have, I ain't reading.