I got this on Blu Ray when i first got a player/hd tv. I've only ever watched it the once though.
I really should give it a second viewing, and see if it improves, i agree that the slasher feel towards the end did kind of alienate me on my first and only viewing so far.
The film itself has some fantastic themes/imagery though, and really stands out. It had this real sense of hopelessness and desperation throughout the film. I like how it's not like other hero films, where its 'let's save the world! rock on!.
You just have a bunch of everyday people, that happen to be suitable for this mission, which for all intents and purposes is just a desperate attempt for humanity to survive. Tbh it's basically a film that asks the question of what do you do, and how do you act when you know the inevitable end of the world is coming?