A Flawed Film
It's not usually considered proper to criticize a film by reference to the book it's taken from, but here we have to. They set out to put the book on film, but they failed to respect the difference between the two forms. (And with Gunter Grass so heavily involved, no one could point out that they were being untrue to the book.)
For those who haven't read the novel, they chopped off the last third, and in so doing they lost the "frame." The frame is that it's ten years after the War, what's left of the family has relocated to the West and had new adventures, and Oskar is telling this story while confined in a hospital for the criminally insane. (Hence the voice-over narration.) We're given enough of an independent check to know that it's not a straight hallucination, but it's clear from the start that we're not supposed to trust everything Oskar tells us; he frequently gives one version of an incident but then backs up and says it wasn't really like that -- though we shouldn't trust his second version either.
You can do that in a novel, but it's very hard to do on film, where you have to put something in front of the audience and ask them to believe it. The filmmakers proceed like they don't realize there's a problem and give us a literal presentation of what was never meant to be taken literally.
The adult, post-war Oskar is crippled with guilt at all the harm he has caused -- if you thought he was nasty in the film, in the book he's worse -- and that colors everything he tells us. When people ask why anyone should care about such a dreadful little boy, they haven't gotten that this is what the adult Oskar thinks of himself, looking back. (Interesting, now that we know Grass himself had some "ghosts.") The film fails to convey that.
It also didn't help that so many people assume that Oskar is making some sort of protest against what's happening to Germany. In reality, a perpetual toddler who screams and breaks things when he doesn't get his own way, even though he's old enough to know better, Oskar IS what's happening to Germany. And looking back after the War, he knows it -- to the extent that he tries to pretend he's not German at all, but Polish.
That's all in the book, but they fail to translate it to the screen. They're too busy putting the book on the screen as a straight story without stopping to show us what it's all about. So I call it a flawed film.