You shouldn't watch this is you haven't read the book... obviously. The story is not quite the same, some characters are introduced or modified, and parts of the story are cut out to fit the running time. It's a decent movie, but it just doesn't fulfill the incredible potential this series had.
Still, if you've read the books, this is pretty cool to watch as a complement. Seeing the characters and story acted out on screen is fun, and some of the new elements are intriguing. Plus, the effects are excellent, and Zooey Deschanel plays Trillian.
In a sentence:
Worth watching if you've read the books, as it's pretty cool to see it in a movie format.
