I enjoyed "The 40-Year-Old Virgin," but didn't feel it was entirely worthy of all the enormous acclaim it garnered. And for all the praise "Knocked Up" received, I thought it was fairly average. So I wasn't expecting much from "Superbad." However, I'd have to say it's the smartest and funniest Apatow film yet. I know he didn't direct it, but his influence is distinct. And in the past I've felt indifferent towards Apatow's brand of humour, but combined with Rogen's sensitivity towards the teen condition here, it works really well.
People have compared it to John Hughes' better teen comedies, but I don't think the approach is entirely similar. "Superbad" is cruder, ruder, a bit more focused on the sex & alcohol jokes. But it works really well because it captures that dynamic - better than "American Pie" because it invests more care into its characters. One of the strengths of "Pie" over other films in its genre (such as, say, "Slackers") has always been its characters, and "Superbad" similarly relies on its characters for support. Plus, it's just really funny.
The performances all around are pretty funny, even if the Fogell/McLovin pop culture references are ruining a good joke (kind of like "Yeah, baby!!" ruined Austin Powers when 20-year-old frat boys around the country thought it would be funny to shout the phrase every five seconds).
Jonah Hill and Michael Cera have great chemistry and another one of the strengths of this film is that its teenage characters actually do look their age (even though Hill is in his early 20s, he still passes as a senior). But it was Bill Hader and Seth Rogen who stole the show for me as the clueless cops.
Overall this is just a funny, well-made film with heart. It doesn't mock its characters or their predicaments but casts a knowing eye on the high school experience. It captures all this very well and was certainly, in my opinion, far superior to "Knocked Up."