To be honest the ending doesn't really add up. And that's the big problem with it.
The film is about paranoia. How one man mistakes everything he sees for danger, and it begins to overrun his life, until he finally snaps and events take a turn for the worse, placing a huge strain between husband and wife.
It's an interesting ending, as I've already said, but it does have holes, and that's where a lot of people will lose interest, if they haven't already done in the first hour. I think it's decent, but I'm perfectly willing to accept that people could find this absolutely brilliant or absolutely terrible, so from that point of view I don't really think this is a film that people should listen to critics for, and make their own mind up about it.