Simply put, it's a mindfuck. Honestly. It was as mentally stimulating/psychologically thrilling as Higurashi no Naku Koro Ni (at least in my opinion, and possibly due to being so similar).
Anyone know a good series which has a deep and dark story? Not horror, a bit psychological like Higurashi, Mirai Nikki, or Elfen Lied, with more social aspect and less action scenes.
I'm getting intrested in those kind of animes atm.
Death Note. You should've already watched this but just in case you haven't, well... watch it :p Monster. A must-read manga. Not sure about the anime but I heard it faithfully follows the story of the manga ;o Steins;Gate Bokurano. Don't get intimidated by the mecha genre, it's more about the characters than the fights.
that's about it from me... If it was about manga then I'd be able to give a looot more :p