If you're pissed at the twist, a second viewing might help. Hear me out.
It took me a second viewing to understand the plot twist. First of all, like many of the IMDb posters pointed out some time back, she is not just an obsessed killer. She is a split personality. Marie was two "people" completely unaware of each other battling it out. On one side, she was the killer, and on the other, she was running FROM (or in the end TOWARD) the killer. But it takes a second look. Because that's where you'll start to see things you didn't see before, and you'll start to understand why the twist makes sense. But the whole film will be completely wasted on anyone who only watches it once, In my mind, it went from complete cop out to clever and mind blowing.
However, putting that into perspective, the second viewing turned around and presented a number of points in the film where things that made sense the first time no longer made sense. Like Marie chasing the truck in the car, or watching a murder from inside the closet. But I just chalked it up to cheap tricks to throw viewer suspicion.
The twist was clever enough, and for the gore hounds, the kills are well done (the unrated "Haute Tension" is the only one to watch). This was a fun ride, and now that I know what's happening, it makes it more rewatchable.