This post is late; I'm surprised how strongly I reacted to this film.
I'll say upfront that I haven't (yet) seen any of Simon Pegg and Nick Frosts' other collaborations, but by the sounds of it they are funnier than this one, and I definitely look forward to checking them out... eventually.
As for Paul, I'll keep the review brief then share a few thoughts about what stuck with me after the movie. A short history: Paul is the result of yet another collaboration (the third, I believe) between Nick Frost and Simon Pegg - although it is not counted in their 'Blood and Ice-cream trilogy' alongside Sean Of The Dead and Hot Fuzz. This is probably appropriate, as it doesn't appear to have garnered such high praise and classic status as those films. This may not be entirely their fault; reportedly there was some heavy studio intervention during pre-production on this film, in an effort to make it more 'mainstream', possibly to justify the cost of the CGI. The end result, however, still seems aimed squarely at a narrow audience of sci-fi nerds. The film is, essentially, a love letter to geekdom. And that's probably how it was originally intended.
Sadly, the film is just above average - with hints of potential greatness in some of it's ideas. It's funny, but I never laughed out loud (granted, I watched it alone). It drags a bit in the middle, although the last 20 minutes suddenly get very exciting. It's loaded to bursting with references to science fiction and pop culture, at least half of which went straight over my head. The way the alien is integrated into the the world of the story is extremely clever - in such a way that leaves the filmmakers free to rip off any alien movies they like, and have it make absolute sense. The concept is so great, and the execution is mostly adequate but there's something just a little off... I think Roger Ebert summed it up best: the problem lies in the character of Paul; he is, essentially, Seth Rogan with green skin.
Ok, so the main thing that resonated with me in the film was the subplot involving Ruth, the stereotypical naive fundamentalist christian girl, suddenly encountering this alien, and absorbing his knowledge in a faith-shattering experience. As a Christian, it's a little disappointing to see my faith on screen being represented as reprehensible, unlikable, unagreeable, narrow minded... The sort of things atheists accuse Christians of being over the internet, when they're more interested in stirring up a fight than having an actual argument. I'm not saying that all Christians are above this, but it's frustrating to see such deep and important issues reduced to Straw Man and Ad Hominim arguments (look them up).
This film has an overtly atheistic slant, poking fun at the extremity and hypocrisy of some christian sects (Jesus shooting Darwin??) and more generally poking fun at the idea of God, in a universe clearly driven by naturalistic mechanisms that have produced life separately, on more than one planet; Paul himself being the clearest evidence for this.
So my first reaction was just to be annoyed. Later, I found myself wondering about the themes of the film. Risk might be one of them; Paul encourages Graeme to 'roll the dice'; to take a chance. I thought it kind of surprising that Paul turns out to be... exactly who he says he is. Graeme puts his trust in him at the start - for no real reason other than to take that chance - and in the end finds his trust rewarded. His faith rewarded. That was it, this movie is about faith. That got me thinking.
Take the character of Paul; a being from another world, who imparts knowledge and demonstrates fantastic powers. He heals the blind, and sets the girl free from her confined life. He's persecuted by authorities, trusted by a few friends. Ultimately, he puts his life on the line in order to save his friend from death and ascends from earth... Need I go on? He's a perfect Jesus allegory! Down to the point that Ruth paraphrases the Bible, saying "Now I see". She means this literally in that her eye has been healed (Jesus reference) and metaphorically in that he has opened her mind to the reality of the universe, delivering her from the set ways of her old religion and, dare I say, freeing her from sin.
But I get the feeling that's not what they were going for. In bringing up God and aliens, though, it seems the film was going for a discussion of faith. These two nerds have a religion of their own, and Paul appears to them like a saviour. I'm sorry! Enough with the allegory. If we just examine the idea of faith from this film, what we see the 'true' faith of the nerds is rewarded completely. Paul is everything he claims to be, and becomes a great friend as well as a cool guy to hang out with. And it's this that makes the film, as almost every reviewer has called it, a 'sweet' film. There's no deception, there's no bitterness or cynicism; just a couple of dudes hanging out, with this other dude with green skin. This is about faith gone completely right. It may be overtly anti-religion, but underneath it seems to carry a different message; sometimes it pays to just have faith.