I know I'm going to regret this, but I'm going to bring it up anyway, because it works here: Battle Royale. I couldn't care less if the movie accurately depicts what happens in the manga. What I care about is if the movie is entertaining and enjoyable to watch. And it is. Hell, even my favorite movie of all time, Apocalypse Now, is based off a novel, "The Heart Of Darkness". Is it faithful to the novel? From what I can gather, no. But I don't care. It's still one of the best films I've ever seen. Same goes for Battle Royale.
It's not that far off base, actually. Aside from the Vietnam War and various modern elements, it follows the same basic plot, and has all the major characters. (Kurtz, Kilgore, whatever the Hell Robert Duvall's name was...)
Are you a verteran? I love that movie, and I've heard that it's even more powerful if you've served in the military.