Up until episode 12 this is a fantastic series keeping you in genuine anticipation, satisfaction and then anticipation again. The action is not the very best you have ever seen but it's okay and there is plenty of it around. The game with the truly exciting special powers makes up for everything though and you'll find yourself on the edge of your seat every time a new technique is getting revealed. These episodes are fairly well written too, featuring an interesting plot and also doing a fine job with character development and in setting up emotional attachment to most characters.
Now after getting you hooked I suppose, indicated by a token flashback episode a very different series emerges. The animation now becomes minimum effort most of the time, meaning a heavy decline on action scenes by definition and a lot of mere shaking of static pictures. This would be party acceptable saying that we'd have arrived at a more talky, story centered bit but unfortunately this is not the case either. The thing just runs out of steam and starts kind of going nowhere, doing that with a critically slow pace too. Ideas barely emerge anymore and everything just becomes very incoherent. Worst is that also the fights and deaths of the characters become disappointingly anti-climactic blowing a fantastic premise right out the window.
Now this doesn't stop me from genuinely recommending those first 12 episodes as they rocks quite hard nevertheless. I really can't recommend you to waste your time enduring the rest as there really is barely anything to look forward to, but maybe just reading the story (fighting those battles in you're head) on wikipedia and then cut to the final episode right away might be a good solution.
I'd give a 9/10 for the first half and a 2 for the rest.
