Zaion is an exercise in lack of thought. Not defective thought like some animes, no half-baked explanations of convienient plot twists, but a nearly complete lack of interest on the part of the writers. The second half begins to redeem the show but it's already too late. Mediocre story, stale characters, and glaring holes in logic have done their damage.
At some point in the past (at least 1 year ago tho no time frame is given) a meteor fell to earth releasing an alien virus that causes the infected to turn into rampaging mutant monsters, infecting anyone they touch. There is no cure so the only way to deal with an outbreak is to seal the area and kill all the infected. A secret organization called C.U.R.E. is responcible for containing outbreaks and keeping the whole thing secret from the world. Their dedication is aparent by the way no one seems to care how it all turns out. The operators for their remote assult robots act like it's a boring videogame ("Finally I'm dead, now I can get a coffee"), the nanite-infused NOA soldiers are given no equipment and are left to kill hoards of mutants barehanded (sure they have superstrength and armor but give the poor guys a grenade or 2 at least), and not only does no one seem to bother using any kind of tactics but in the end when they start, they get arrested for it (for disobaying orders, yes. but still).
All of this is meant to be backdrop for the mild romantic story of Yuuji, a NOA soldier, and Ai, C.U.R.E.'s new superweapon (who creates a 40' hologram that hugs things to death. how this is supposed to be the ultimate weapon to combat a virus is anybodies guess). It starts off poor, as no character has any personality until the second half, but gets more interesting toward the end.
Nothing good or bad worth mentioning
Nothing spectacular but nothing too bad either. Except the CG robots. They're as generic as possible to make things easier on the animators and were very poorly meshed into otherwise hand drawn fight scenes. Mostly they don't even move except to walk forward until the big fight in the 3rd episode. They barely move while getting jumped and smacked around by guyver-esque mutants while then fall over at the wrong time in weird directions like someone's poorly done greenscreen fight.
One of the worst parts of this show. All the characters and completely lifeless until the second half. After which point most of them die as soon as their personallities finally come out. Even the 2 mains only stand out because they are the focus of the story. If not for that, they wouldn't even be memorable.
For the first half I found myself with nothign better to do than pick though all the strange logic that presented itself. But once the story began to come together and the characters started to gain personallity, I found I began to like the show. Zaion does finally manage to leave that pleasent feeling from the happy ending (though the ending itself presents another hole in logic if you're already thinking about such things)
Not very good, but some will like it anyway. And it's short enough to watch in one sitting.