Now I've got to say, when I watched it the first time around in 2009 I was confused, so much so that I started rewatching bits to see if it they made sense after learning the twist and they did so I was amazed by the fact that they pulled it off.

However rewatching it now, knowing full well what's going to happen has dropped my originally high opinion quite significantly. It does all still make sense but in places the dialogue feels unnatural from that point of view.