Personally I hated it.
I thought the story was horribad and the main two characters were not likeable at all. Also, since I watched this anime for the first time only 2-3 years ago, the concept of terrorists disseminating their messages via viral riddles on a Youtube-analog already seemed quaint and outdated to me.
It had very good art direction and animation, though.
I'm curious, do you watch the raws? How good is your understanding of Japanese?
Or are the early releases dub/multi-dub?