I haven't watched many anime's yet but so far this one beats everything i've ever watched. It is kinda similar with Elfen Lied as they say but i believe it's unique. I watched both seasons in 2 days so this gotta be something right?
Plus the whole atmosphere just makes you stunned and you cannot turn it off. This is absolutely one of the best mystery / horror and if you count the second season then also Sci - Fi anime's that has ever been published!
Arigatou~