The first person I know of to see The Simpsons Movie, Brian Lowry of Variety, has a mostly positive review:

After 18 years and 400 episodes, "The Simpsons" has developed a wide array of potential moviegoers, from those who still watch to those who once watched to those who don't watch anymore but now have kids that do. The question is how many will feel inspired to ante up for something so readily available for the price of enduring commercials and Fox's incessant on-air promotion. Happily, the long-gestating movie itself offers a fine incentive, and Fox's inspired marketing campaign (7-Eleven becoming Kwik-E-Mart? Genius) should ensure enough curiosity to stuff the studio's pockets, as it were, with dollars from doughnuts.

Put simply, if somebody had to make a "Simpsons" movie, this is pretty much what it should be -- clever, irreverent, satirical and outfitted with a larger-than-22-minutes plot, capable (just barely) of sustaining a narrative roughly four times the length of a standard episode.

Spolier warning: The plot setups are described in broad strokes.