Knocked Up is a silly yet serious comedy. You have a mix of the completely goofball and eternal teen played by Seth Rogen, and the very serious and proper career oriented young adult played by the beautiful Katherine Heigl. You couldn't have picked better opposite.
The whole movie is like that and makes you alternate between the two world. How they mesh is pretty simple, you'll see. It's very touching, and you'll be rooting for all parties, because they are all right and wrong.
There's something for everybody. Emotions, laughters, completely idiotic behaviors, responsible decisions, breakups, makeups, kisses, tenderness, and sex. That movie gives you everything and is the opposite of the dating movie, it goes backward.
One thing irked me though: at some point, it seemed to be anti-abortion. I'm pro-choice, and that ruffles my feather. Beyond that, it's a crazy setup.
The acting is good, mostly silly, the story is excellent, if nothing new, and the soundtrack very appropriate.