The first thing I'll say is that I do recommend watching this, despite giving it just a 6. I would actually rank it 7, as my enjoyment of it was about 7.5/10, but I honestly can't do that as a watcher nor as a critic/analyzer, as the art, sound, animation and drawing style isn't anything outstanding nor special, and the story is pretty...
Weak. That's the word I would use, but not a word that does it justice. You see, let's break the story further into smaller elements : first logic and consistency. They suck here - the motivations of half the characters are really realistic and well defined and executed, and the other half is illogical and full of plot holes more than Swiss cheese on roller blades.
Next is the idea and setting. These are pretty good - nothing special but definitely interesting. Overall this means the story is slightly generic, and seems to hit a snag on one enemy side, which seemed to be very poorly thought through thus it falls apart quite a bit, but not to the point it's not enjoyable to watch.
What does make it interesting and worthwhile then? Again, said idea and world setting, but mostly - the characters and slightly unconventional way to tell the story!
There are THREE DUOS of main heroes (which I normally don't like due to it usually meaning each barely gets screen time, but here it actually works great!) , and a few other secondary characters that are almost as central. You're mainly introduced to the world at first through the typical cliche confused shounen hero and his typically vaguely defined 'person I want to protect', friend, childhood friend and love interest in one package. Then the focus shifts more, and even completely to the other two pairs, and by the end the three start to juggle quite masterfully while interacting in fun, quirky ways.
Now then that's about as much as I can say without going into details and spoilers, and that might not sound like much to go on, but I assure you said characters and their interactions ARE interesting, and anyone who read 'roadside picnic' or any similarly structured book will know just how much a shift in perspective from one character to the other can contribute to enjoyment, especially when the story's plot twists aren't a part of the said shifts from one story teller to the other.
Final score ; 6.3, would definitely recommend, not as a must watch, but definitely as not too bad a piece.
