One thing readers love more than anything is to learn that a book they love is going to be made into a theatrical film. It's a form of validation, isn't it? ("If Hollywood saw fit to finance this film, then it must be because the story is that good. Well, I could have told them that!") Seeing that a few suits are willing to gamble some money on a book we like makes us feel good.
Read the last SF Signal at Kirkus.
But sometimes readers are disappointed with film adaptations, not necessarily because the film is bad, but because it interferes with their own perception of the book. ("They got George Clooney in my Batman!" Or "they left out important characters and entire scenes!")
To be fair, film is a different medium than paper, and storytelling must naturally change as part of that transfer, but that doesn't mean the film cannot be just as enjoyable as the source material. That is, unless we're talking about the following 7 Sci-Fi films which simply do not do the source material justice.
Dune by Frank Herbert
Herbert's Dune is one of science fiction's most beloved novels, so it was a natural choice to be adapted to film. Can you say "built-in fan base"? The problem was that the 1984 film was a mess, so much so that director David Lynch (yes, that David Lynch) used the eponymous pseudonym Alan Smithee in the credits. One problem with the celluloid version—the many internal monologues of the book did not work well onscreen. (There was a TV miniseries adaptation back in 2000 that fared better, but was still inferior to the novel.)
I, Robot by Isaac Asimov
Asimov's novel is actually a collection of related short stories about robots and a logical examination of his famous Three Laws of Robotics. The movie? Not so much. It was a rewritten version of an entirely different film, tangentially linked to Asimov's work after the fact—a modification from which even the star power of Will Smith could not recover. (See also: "Bicentennial Man", yet another of Asimov's wonderful robot stories poorly adapted into a mediocre film starring Robin Williams.)
Lathe of Heaven by Ursula K. Le Guin
If you only saw the 2002 made-for-TV film Lathe of Heaven, you missed out on the eloquent prose of Le Guin, whose insightful observations led to thought-provoking ideas in her outstanding novel. This adaptation lacked most of that. (A 1980 adaptation starring Bruce Davison, which I have not seen, is said to be much better.)
Millennium by John Varley
Varley's novel, in which humans in an over-polluted future raid the past for the healthy people needed to rebuild civilization, is both original and mind-bending. You probably never even heard of the plagued 1989 Kris Kristofferson and Cheryl Ladd film, which underwent so many rewrites and Directorial changes, you'd think someone was time-hopping into the past to change the film for the better. And failing.
Starship Troopers by Robert A. Heinlein
While Paul Verhoeven's 1997 silver screen gore-fest was not bad (in a popcorn-action flick kinda way), it simply didn't do the source material justice. Heinlein's 1959 military science fiction classic, which portrays a new recruit's rise through the ranks as he fights alien bugs for the right to vote, was simply a better examination of moral and philosophical issues.
The Day of the Triffids by John Wyndham
There have been several adaptations of this classic sf work, most notably a 1962 schlocky film that was essentially a monster movie with man-eating plants. The book, however, focuses less on the leafy-but-dangerous plants and is instead a subtle yet unsettling "cozy-catastrophe" in which most of the world is rendered blind and humanity tries to rebuild.
The Hitchhiker's Guide to the Galaxy by Douglas Adams
Another of science fiction's most beloved works, this one noted for its effective humor. Not only does the story of the 2005 film differ from the book—not itself an unrecoverable crime—it makes the absolutely unforgivable mistake of not being terribly funny. Perhaps some of the other many adaptations (including radio, albums and stage plays) fare better.
John DeNardo is the editor of SF Signal, a group science-fiction and fantasy blog featuring news, reviews and interviews. He also like bagels.