Up to between chapters 280 and 300, I'd give the manga a fantastic rating. The art is good, the plot is good, character development is good. It keeps me entertained and wanting more. Then it falls off a cliff. Between chapter 280-300, or maybe even the start of the final (3rd) phase, it feels like the author just wanted it to end. He also seemed to force the romance between the main character, Kurono, and his girlfriend Tae, even though the vast majority of fans (including myself) despised this as it detracted from the plot. In fact, the entire final phase (the final invasion) is simply to get Tae back - forget his missions, the Earth, and all of the other people he loves, he just has to save Tae!
But the worst part, which is what utterly ruins the grade of this manga, is the ending. The final chapter (named The Final Chapter) is a short than normal chapter, and answers few if any questions. For those that have seen the anime, I would compare it to to the anime's ending if not worse. The author goes from the final battle to the end of the manga in half of a chapter, a quarter of which is simply crowds cheering and Tae crying.
Overall, if you're ok with an absolutely abysmal ending then this is a great manga. I would argue it should have ended after the Osaka arc, which would have been a great ending albeit still with questions unanswered.
