I sort of knew the gist of what makes the film tick before watching it (who'd have thought at almost 15 years after it debuted), but I think it still holds up as a pretty good film.
Don't expect a horror, don't expect just a thriller or puzzler. It's better than that. It's about humans, and even if you guess "THE TWIST", it's still worth paying attention to.
It's not about the surprise, it's about why the surprise and like all good films, leaves the answer to you to determine.