An alien that talks and acts like ... Seth Rogen. But no, Paul is far beyond that. Like Galaxy Quest, Paul is a movie very specifically made for sci-fi nerds like me. And very brilliantly so. So if you're well-acquainted with every scene, dialogue and soundtrack clip from the original Star Wars trilogy, Close Encounters, Indiana Jones, E.T., Aliens, Back to the Future, and so on, this is definitely the movie for you; because the homages are subtle, intelligent, and funny. If not, don't even bother; you won't get it.
PS: Thank you, Mohi, for the recommendation. I wasn't planning on watching this.