The series starts off really well- but slowly goes downhill from there. It starts off as a school story with action mixed in, and then slowly turns to pure action. But that's not the bad part. The action was alright. However, around episode 15, the series went from having a plot, to complete and utter nonsense. The plot becomes incoherant at this point, and the only thing that would save this series would be the sweet tale of friendship between Marin and Melan- if the series itself wouldn't have become borderline pedophilic at this point.(Just trust me on this- it does. >_<;.)
Avoid this at all costs- the anime, anyway. If you're interested in the plot, read the manga. It's the same thing, only without the pedophilic parts, and with a coherant plot.