It's an odd thing; written and directed by Joe Cornish, and featuring (albeit in a minor role) Nick Frost, I was expecting this to be a straight up comedy, with a literate knowledge of the genre it's spoofing, like Sean of the Dead. And while it's not definitely *not* a comedy, in as much as some of the characters are funny people, and others are funny by virtue of being fish out of water in the situation they find themselves in, this is much more of an Alien movie than a comedy.
The situation is, on her way home from work one night, a young nurse called Sam is mugged by a local gang of youths. During the mugging, something literally falls from the sky next to them, and destroys a car. On investigating, the youths find that it's some kind of weird beast, which attacks one of them, then runs off into the night. They pursue it, corner it, and kill it. They take this thing, a weird, grey, slimy eyeless thing, all teeth and claws, home, because they think that it might be worth something. However, further things are falling from the sky. Much bigger things, which seem intent on attacking the block of flats that the youths (and the young nurse) live in, and the inhabitants have to form a wary alliance to defend themselves against Rampaging Killer Beasties From Outer Space.
First off, cards on the table, skip to the end - it's a really good film. Reminds me of your low-budget 80s creature features of the CHUD/Critters type, but with a very healthy dose of British wit and style. I think the characters are well observed; the gang of youths are charming, amoral kids, with no qualms about petty crime, and an air of injured innocence about being hassled by the Police, despite actually being habitual criminals. This doesn't stop them being brave, likeable characters, willing to stand up for them and their own. Likewise, the mistake of making the group bond out of necessity never comes too easily, or too deeply. It's very well judged.
The creature effects are lovely; one of the character puts it best, they're "gorilla-wolf motherf*ckers", bounding about the place with night-black fur that makes them difficult to make out, and eerie-blue glowing teeth. If I have a criticism, it's perhaps that these creatures aren't quite badass enough; a few of them get killed in some quite ignominious ways, which in some ways takes away from how scary they are. On the other hand, by the end, there are a *hell* of a lot of them.
So, really, it's quite peculiar, being a teen creature slasher film, but with some honest-to-god social commentary in it. And for my money, if something can be both peculiar, and really good, that film is worth taking time out to see.