This anime really grew to me a few weeks after I finished it. Don't get me wrong, I loved it while watching this series, but this is one of those rare cases that got better over time.
It's best described as a Drama / Action / Supernatural anime. It starts with great action (the first episode was what I hoped Canaan would be, awesome fast-paced action) and works hard on the character-development once you're a few episodes in. But really, with an OP like this, I was already sold at the very beginning:
Check out the full version as well, I think it's even better.
This is the prologue to the Ga-Rei manga, and gave me all the motivation I could ask for to start reading it. Don't be expecting a masterpiece, Ga-Rei -Zero- isn't flawless, but fans of good character development, action and drama shouldn't miss out on this beauty.