This show is all around a very cleverly written anime. It has romance without it being in your face mushy. it has angst but at an understandable level. Drama without getting too wrapped up in it. The characters are deep, and action that only seems to get eye catching artistic when combined with beautiful animation.
The shows plots are complicated, and when the shocking twists that you don't see coming get thrown at you, they are believable and you really accept them rather than coming off too far fetched. It makes you think but it doesn't take a rocket scientist to figure it out.
When you have humans going up against seemingly immortal monsters, it keeps you on your toes and you can't wait to find out how in the world they get out of these messes.
The only keys to unlocking the secret of reclaiming their old bodies is Science, spiritualistic philosophy and the unknown and while these forces seem to be at constant war with each other, when they come together, you're left with a feeling that it all makes sense you just couldn't see it before.
While it doesn't do all of the things I've mentioned up above perfectly. it does them well enough to give this anime an A in my opinion.