If I were to name my ONE favorite manga. It would have to be this one. And for someone who has consumed over 300 series, it is quite the statement. Why is it so good? Let me tell you.
Story: At first the premise is nothing groundbreaking. Save the world from being destroyed. Literally. But as it turns out, the REAL story is actually saving the world...to destroy it. Now that's a story I want to read. Other than the basic premise, however, the pacing and events are quite good by themselves. Let me warn you now. This is a not a "every human a survivor of impossible odds", this is a "every human a human." That is, your favorite characters CAN die, and more likely, they WILL. Mizukami is not afraid of killing off our heroes for the sake of a better story.
Art: Unique is it's own style. Honestly, there is nothing to complain about it. It's not as detailed as dark and murky seinens and not as empty as typical shounens. There is a balance between the two. There are no annoying or unnecessary fanservice shots, which is a huge relief.
Characters: Oh what even to say. Our hero is a dangerously loyal servant of our heroine. He is smart, quirky, (relatively) confident in himself, and even cynical at times. Not your typical shounen protagonist. Heroine is outrageously powerful, outrageously aggressive, and outrageously childish (at times). She wants to destroy the world and she is fully capable of it too. Every other character has it's own charm. The "side" cast is extremely well developed, even ones that only appear for several chapters.
Overall: A unique and highly satisfying read. This story runs from start to finish with little lull time. Mizukami cuts out all the fat and lard and shows only what matters. I don't reread mangas, but I reread this one three times. If you like action and adventure, read it. If you like comedy and drama, read it. If you like ecchi, look elsewhere. But if you are looking for a less-serious but not overwhelming seinen experience, definitely pick it up.