As the father of a 3yo w/Down syndrome, I kind of cringed when I first heard about the movie. The Special Olympics did endorse the flick, though. It supposedly does a good job of breaking down the stereotypes of not only people with Ds, but other disabilities as well.
If you find the Farrelly Bros. funny (which I do), it's their pretty typical, irreverent fare. If you find them offensive and childish, you're probably not going to like the movie.
It looks like it will be pretty close to the South Park episode, where Cartman tries to win by pretending to be "special". Doesn't work of course, and is a pretty funny episode.