I'm surprised at so many bad reviews for this film. Yes, it's not 'high art', yes you can pick holes in the plot, but it's an entertaining twist on slasher films (a genre I personally find very boring in its usual form and I nearly didn't watch this as I thought it was going to be one)that is actually quite original in what it does.
I liked the way it allowed you to see what was really going on from the start without trying to have a 'big twist' at the end which wouldn't have worked as well.
It's worth it for the last half hour alone which, apart from some budget looking CGI, presents a very entertaining slice of mayhem.
A plus point is it's not obvious exactly how it will end and they ending they chose is satisfyingly atypical and makes a refreshing change from the hackneyed 'wounded survivor staggers off into the dawn' kind of endings you get so often.
If you're a fan of straightforward slasher films, there's a good chance you won't like this. If you like something a bit more off the wall, it's well worth giving this a go.