Spoiler Level: Medium
Oh, man. I laughed so hard I nearly passed out. No lie.
Paul is a great movie because it works on two levels. One on level, there's the obvious homage to Steven Spielberg's Close Encounters of the Third Kind and E.T., and to the fanboy subculture in general. There so many in-jokes I could never count them all. A few zinged over my head, but not many. (Or not enough, some might argue.)
But unlike Free Enterprise, for example, the film doesn't require you to get them. Many of them are played in such a way that even if you don't get the reference, the way they're delivered is funny all on their own. (Okay, Higher-than-Medium Spoiler Level here as an example. Highlight it to read it.) For example, when Blythe Danner faces off against Sigourney Weaver, she exclaims "Get away from her, you bitch!" Out of my group, three of us laughed because it's Sigorney Weaver's most famous line from Aliens now being directed back at her. One of us laughed because it was Blythe Danner being portrayed as a little old lady, and here's this little old lady taking charge and kicking butt. And there's numerous scenes like that. I remember finding Spaceballs disappointing because I felt Mel Brooks explained all the jokes. The jokes aren't explained here, so either you catch the reference or you don't-- but even if you don't, the scene has been crafted in such a way that the line is funny all by itself. It's the perfect solution, and one that could only be achieved by people who genuinely love the source material they're lampooning.
And then there's the characters themselves-- Simon Pegg and Nick Frost as two Brits dealing with America for the first time, Seth Rogan as (the voice of) the rude & crude title character Paul, Kristen Wiig as Ruth Buggs, a devout Christian who has her world turned upside down. Not to mention the people who end up chasing them, including Ruth's father, government agents, and angry rednecks. That's the film's other level; it is, first and foremost, a flat-out comedy. You could never have watched a single frame of science fiction in your life and you'll still find a lot to laugh at here.
This definitely goes in the list of top fanboy movies. Come to think of it, the only other two on the list are the brilliant aforementioned Free Enterprise, and Jay & Silent Bob Strike Back. And come to think of it again, I don't think I've laughed at a movie so hard since Kevin Smith. And that's saying a lot.