Starship Troopers was a odd one for me. I love the book and the film was disappointing for straying so far from the source material - but it's a terrific film in its own right so I like watching it.
If you want a film that's close to the book in most of the important areas and really gets the "spirit" of the source - try "Manifest Destiny". The book (a short story or novella would be more accurate) is a gem - the film is almost as good and quite surprisingly so considering that it's really quite a "deep & meaningful" story with 90% of it just having two characters on-screen.
