Film novel
Doesn't mean that the movie "sucked", though. It's extremely hard to find movies that are 100% faithful to their book origins, even ones that are as successful as, say, the Lord of the Rings Trilogy.
Of course, for every LotR series (or even Harry Potter series), you have...things... like Twilight, where you wish they weren't as faithful to the books as they are.
If you had never read the novel, though, then Starship Troopers would easily be classified as a successful summer blockbuster. And even if you have read the novel, just remember there's a lot of events that go on in the novel that get very little attention in a book setting, but would require a lot of onscreen time... like the various invasion sequences. Not to mention the invariable complaints as to whether the "Mobile Infantry" suits were "good" adaptations of the book's descriptions or not. Irregardless, considering that my wife enjoyed it, & she's not exactly a sci-fi fan (horror, yes; monsters, somewhat; serial killers, especially cannibals, cue Mr. Burns saying, "Excellent!"), I think we can call it a success. #2, not so much; can't say on #3, haven't seen it yet.
Of course, it could be worse. It could have been the complete opposite, where the movie is much better than the book. I remember reading the novelization for the first Christopher Reeves Superman movie. Don't remember all of the details, but I do remember that not only was the nuclear missile issue completely missing from it, but the 2nd half involved Lex Luthor & Superman traveling to an alien planet (Luthor using a spider-shaped spacecraft), and having to defeat an alien version of the Pied Piper that nearly took over Earth. *shudder*
