You'll know in the first 10 minutes or maybe as soon as the familiar FOX trumpet flourish sounds. We meet the yellow, manic, elastic residents of Springfield which is where? right in their midst, and they never let us out. I'll let the story unfold for itself, but just a few innocuous words: The lure of the donut serves as a key plot device; we get the most epic Itchy & Scratchy Show ever; and when President Schwarzenegger says, "I miss Danny DeVito," you'll know why.
The screenplay is credited to 11 writers, including Matt Groening, James L. Brooks, Al Jean, and Mike Scully (there's no sign of Sam Simon anywhere in the credits). Suffice it to say Homer Simpson has imperiled the existence of his storied hometown, and only Homer Simpson can save it. It's not giving anything away to say that Homer plays the fool to Marge's pragmatist, Bart has father issues, Lisa has a crush, and Maggie continues to represent all that is pure and wise.
Reports from early test screenings indicated the film was strong in its first two-thirds and softened in the last section. Only those lucky test screeners will know what, if anything, has changed since then. It is true that the rapid-fire sight gags predominate in the first half or so, but all the trademarks of the show, including the references, self-references, and meta-references, ride shotgun to the story throughout the movie's 89 minutes. The filmmakers take aim at the scale and sweep of the Big Summer Movie. But like the show, The Simpsons Movie is strictly whimsical and irreverently moral. Every time it approaches self-importance, it promptly undermines itself with ruthless efficiency.
According to The Simpsons' extensive no, exhaustive Wikipedia entry, President George H. W. Bush once promised "to strengthen the American family to make them more like the Waltons and less like the Simpsons." Thankfully that didn't happen. Say what you will about Rupert Murdoch trampling on the Wall Street Journal's editorial independence; at least the NewsCorp baron gave Jim Brooks a contract that forbade FOX from interfering with the show's content. Even after 400 episodes, we still enjoy our FOX gags (in The Simpsons Movie the obligatory digs come early).
What works about The Simpsons on screen is what always has worked about the show: Despite the fact that we're sitting in a movie theater, we still feel like they're in our house. They are us. The only show with more longevity is Gunsmoke, but unlike Dodge City, Springfield is nowhere precisely because it's everywhere.