This show is one of my absolute favorite animes. It's a fantastic mix of being dark, and sad, while at the same time retaining enough silliness and action to balance out nicely.
The show takes place before the manga series Ga-Rei, and revolves around two young female exorcists, Yomi Isyama and Kagura Tsuchimiya.
The story is powerful and high-action a lot of the time, and it has an amazing opening theme and ending theme.
I highly recommend this show to anime fans.