Ben Stiller isn't that funny in it, but he's not really supposed to be. He is, more or less, the straight man upon whom all of the hilarity happens around. Matt Dillon is hysterical in that flick, and the supporting cast is damn good (Chris Elliot, the guy that plays Tucker, etc.). So many classic lines in that flick. Overrated? No way. My brother and I saw it in the theater when it first came out, and the whole place was roaring with laughter throughout.
Honestly, the only movie I've seen where Stiller actually is really funny was Dodgeball, and that was largely because he was playing such a ridiculous character.