Lovely Series!
This series comes off as a little bit childish at first-I mean, how many childrens series have little guardian-like characters that help humans or battle each other? Pokemon and Digimon come to mind, as well as many mangas that deal with out-of-this-world encounters.
But this series defies the norm-it may seem childish, but it actually has more mature topics and themes running underneath the series, while still staying at a level where younger readers can read it without their parents freaking out. It deals with loss, deception, first loves and true loves, friendship, trust, and of course the good versus evil conflict, all while maintaining a great storyline that doesn't become too crowded with extras.
I am a huge fan of the art of Peach-Pit, and this is one of their many amazing series. I reccomend looking at all of their series-if you are at the appropriate age level, of course. :]