I'm not going to give away the ending - it's a good shocker, but I won't give you anything else. It's just too cool to ruin for someone who hasn't seen it. The animation is extrodinary - since there isn't anything really supernatural going on here, the artists can't go overboard on anything. Instead, they tastefully give us realistic people and backgrounds. And that's harder to do than anything else.
Background music is also well suited to the mood.
It isn't for everyone, but if you like psychological horror, this is definitely a must-see.