To be honest, after seeing it the second time, it's probably going to be one of my favourites of the year. I think it's impeccably put together, the music, the structure, the b-movie homage, the acting, the cinematography. Watching it knowing the whole story every scene, the acting, the directing choices, everything had a new depth to it.
This film isn't about the twist at all, it's about the story. It had much more emotional impact on me the second time too. I think people expecting some twisting thriller are bound to be disappointed, but the more I think about this film the more I believe it's utterly brilliant.
Edit: Also I'm sure I read a review that mentioned it being two films. The one you see the first time, and the one you see the second. For me that was absolutely true, the first time, I had an idea of the basic twist, but it kept me guessing, kept me wondering, and worked as a thriller. The second time, knowing exactly what it was about, it became something else entirely.