Having seen how split the reviews are, I feel moved to write my own.
Lots of other viewers found it hard to engage with the characters because of the nasty mugging the commit at the beginning of the film, which is quite understandable, and perhaps if I'd been in a different mood when watching, I'd have felt the same. However, I think that the character arcs that they go through are the real strengths of this film. I've grown bored of films where the characters seem set in stone as 'goodies' or 'baddies'; Hollywood is the worst offender, but plenty of Brit-flicks do it too. The world isn't like that, people are complicated, and the script and acting are strong enough to make the progression of the characters believable.
Add to that that I love monster movies, especially when the CGI is not overwhelming, and the coarse humour gave me a lot of laughs.
There is some social commentary in here, but the film doesn't beat you round the head with it. The message I took was basically '15 is a bit young to be beyond redemption'.
Finally.. to use the complete unknown John Boyega as the lead was a risk that paid off brilliantly. I look forward to seeing him in bigger and more serious roles soon.