I want to start by saying two things:
1) Despite the fact that all of the major characters in this movie are teenagers, this is not a kids' movie. For all of the parents out there -- don't drop your 13 year old off at the mall to see this one. "Sexual situations" and "foul language" (both of which I'm all for, by the way) aside, there is a fair amount of nudity here, as well as other things you probably don't want your junior-high kids seeing in a movie theater.
2) This movie was written by guys. It is about guys. I'm not saying that women will not enjoy it -- they will. They will laugh alot. But guys will laugh more, and they will often find themselves embarrassed by how closely they identify with one character or the other in a given situation. And then they'll laugh some more.
Okay, that's out of the way.
This movie is goddamn funny. From the first moments, I laughed my ass off. The story follows 4 high school boys getting ready to graduate. The four friends, though they are all from slightly different "cliques," make a pact to help each other get laid before graduation. along the way, they learn about themselves, and a hefty bit about masturbation and the internet. Pretty standard teen movie fare, I'll admit. In fact, when the movie isn't ridiculously funny, it is sometimes boring. But that rarely happens for more than a few minutes at a time, and then it is back into the extreme belly laughs.
Perhaps the weakest part of the film is not the somewhat over-used plot, but the poor acting that plagues much of the supporting cast. With the exception of the 4 leads and Eugene Levy, the acting was seldom as strong as the script and, in one notable case (the "choir chick" girlfriend of one of the principles), downright awful. Fortunately, the four principle actors (Jason Biggs, Chris Klein, Thomas Ian Nicholas, Eddie Kaye Thomas) and Eugene Levy are so strong that the poor acting behind them matters very little. Casey Affleck (yes, Ben's little brother) has a nice cameo, but the part is really too small to add much.
I highly recommend American Pie. I've specifically not given away any laughs, but trust me, they are there in spades. It is a fine, funny movie and, if you can get over a weak supporting cast, you'll definitely enjoy yourself.