3001: The Final Odyssey by Arthur C. Clarke: Though Clarke is one of the most respected sci-fi authors ever, and this novel continues the saga began by the iconic 2001: A Space Odyssey, Redditor EveryGoodNameIsGone termed this book "utter garbage."
The Saga of Seven Suns by Kevin J. Anderson: Anderson has built quite a career both as a solo author and collaborator, but this epic saga has some outspoken detractors. Redditor FlaveC called it the "worst pile of sci-fi crap ever."
Stranger in a Strange Land by Robert A. Heinlein: One of Heinlein's best-known works is also one of his most divisive. Some readers can't get enough of it, while others -- like lexabear -- couldn't abide much of it, describing the second half as "'and then they had sex a lot' for half a book."
Forever Free by Joe Haldeman: Though The Forever War was highly acclaimed, its sequel isn't getting as much love. Redditor giltirn said it "jumps the shark into cloud cuckoo land within the first chapter."
Brian Herbert and Kevin J. Anderson's Dune novels: After Frank Herbert's death, his son Brian opted to continue the legendary Dune saga, but some readers weren't impressed. Redditor kengou called the Brian Herbert books "horribly written."
Battlefield Earth by L. Ron Hubbard: Though few would argue that the film based on this epic sci-fi tome was terrible, the book has many fans, but there are still readers willing to name it "a piece of schlock," like Redditor ikidd.
Deathstalker by Simon R. Green: Green's Deathstalker saga has been successful enough to continue for nine books, but some readers complain that the first volume is too much like a fantasy novel. Redditor Maldevinine called it "incredibly frustrating" if you were hoping for a sci-fi novel.
Dhalgren by Samuel R. Delany: Though it's one of Delany's best selling novels, some readers simply can't get through it. As Redditor ikidd said: "If you can get through Dhalgren, you're a better person than me. And I like Delany."
Dies the Fire by S.M. Stirling: Though Stirling's novel drew a large amount of critical acclaim upon its release, some weren't too big on its concept. Redditor muskrateer said it was "like Revolution done worse."
Ender's Game by Orson Scott Card: Though this book has been read by millions of high school and junior high students over the years, some feel it doesn't quite work once you've grown up a bit. "It was a great book when I was 12. Not so much now," said Redditor Chtorrr.