Paul has a terrific idea, and while the opportunities for greatness are evident, the writing team that gave us Shaun of The Dead unfortunately missed the mark here; although there are still several laugh out loud moments and enough pop references to just about every sci fi film and TV show ever written.
The film starts with a very nice intro, showing us the 1947 event that sparked all of the UFO lore. From there we fast forward to a couple of nerdy Brits, on vacation and taking in a huge comic book convention in San Diego. The pair, both sci fi aficionados, decide to do the UFO pilgrimage - hitting Area 51, the black mail box and then Roswell - with a hilarious side trip to the barren wasteland where the famous Star Trek episode The Arena was filmed.
While at the AlienInn at area 51 they are accosted by a couple of red-necks, and while making their escape, they back their RV into the rednecks truck (this is funny because when they later meet up with the rednecks, they do it again!). As they are high-tailing it down the road a set of headlights comes up on them fast. The Brits fear it is the rednecks, but relax when they are passed by a Crown Vic (preferred car of the secret service). The Vic then spins out of control and crashes into the sagebrush. When the pair investigate the crash scene they meet up with a brilliant bit of CGI - the prototypical alien - a smart mouthed, wise cracking dude who has evidently escaped after 40 years in Roswell. The pair agree to help Paul get to a rendezvous point so he can be picked up by the mother ship, and therein starts a rather bizarre and often quite funny road trip.
As they head north, the secret service is hot on their tale, egged on by the voice of a female commander who keeps threatening to "send in the big gun". The big gun is a wonderful surprise at the film's conclusion, but it is with the ss agents, especially the two rookies, that the film makes its real blunder - making the young agents total buffoons. This doesn't add anything remotely funny to the film, which would have been better served with all the footage of them being doofuses lying on the cutting room floor.
Along the way there are some pretty snarky rips at fundamentalist religion... if you're a fan of Bill Maher then you'll howl with laughter - if you're a "believer", not so much - though I suppose you might think that the tee shirt with Jesus shooting Darwin is awesome... for all the wrong reasons.
In the end, this is a nice, funny bit of pop culture - but I doubt it will reach cult status as the aforementioned Shaun, or Donny Darko. It's glib and has a certain tone, but the extended chase scenes, especially with the fundamentalist dad determined to keep following the Brits because they "brainwashed" his daughter, are pretty transparent... just a way to set up a joke concerning the Lord and the laying on of hands. As Paul states when dad says "god be with you"... "Yeah, whatever".