Besides being a great movie (more on that in a sec) "Knocked Up" uses music wonderfully, from pop to Loudon Wainwright III to maybe the best use of The Clash's "Police on My Back" ever in a movie.
Refreshingly, the movie doesn't call attention to the songs. There's no "I will now sell three copies of the new Beta Band album" or "The Shins will change your life" moments. The music just enhances the scene. Even when Paul Rudd's character wears a shirt with the cover of Tom Waits' "Rain Dogs," it fits his character and the situation perfectly.
The movie itself is a joy. If writer/director Judd Apatow ("The 40-Year-Old Virgin" and the sadly short-lived TV shows "Undeclared" and "Freaks and Geeks") isn't the next Albert Brooks, he's certainly Brooks' best student. The movie is constantly hilarious, never settling for the easy joke or slapstick stunt — a true feat considering that pot is nearly a character in the movie. But Apatow also hits that off-kilter, real-life feeling of relationships that Brooks did so well.
At one point, after the main characters get pregnant and fail to make a relationship work, Seth Rogen's character calls his dad, nearly in tears, nearly begging him just to tell him what to do. It's a simple line, and there's comedy in it. But anyone in their mid-to-late 20s knows just how deep the truth in that one scene goes.
"Knocked Up" is a different kind of adult comedy: one that teens and college students will find funny, but they'll find a lot more in it once they're older.
P.S. ... DON'T put "knocked up" into a Google images search with the filter off while you're at work or around kids. Just FYI.