A short explanation of the subject title... Although there are plenty of more extreme series that aren't considered adults only (Itokoi Chidori and many others, for example) such series are generally placed in the adults section of bookstores and so they're sold separately from the more mainstream titles. This, however, as an Akita Shoten series, was sold alongside the (rather large) publisher's other series. As such, this manga can probably be considered one of the most extreme series ever released in its age category.
The story is told in a somewhat non-linear way and kind of throws the reader right into things to begin with. And, um, wow. It starts with a giant robot fighting against a giant, anatomically correct naked woman with huge boobs. So it starts with a lot of boob flopping and vagoo flashing (no, seriously), then it ends with a flashback or fantasy (it's ambiguous) of the adult woman pilot of the robot engaging in very graphic lesbian actions with the elementary schooler co-pilot. So, after this adventure, we're taken back to the beginning of the timeline and the next couple chapters are full of completely uncensored pubic hair and anuses, a flashback in which we learn the elementary school girl has a fixation on pubic hair, a barely censored gang rape (complete with penises) of said elementary school girl, and finally, more lesbian scenes.
Now, you may find it odd I'm focusing almost entirely on the T&A/sexual aspect instead of the story. But the truth is, it's so overwhelming that it's what I took away most from the series. The story isn't even particularly bad or anything. I mean, I wouldn't call it amazing or super unique or anything, but holy cow there's lesbian sex and pubic hair everywhere.
But okay, let me talk about some other aspects of the manga. The story is decent. It's a robot story set in the future and they're fighting aliens or something. It's not particularly memorable but it allows for some cool scenes and some of the ideas actually aren't too bad I think. The art is quite good. I'm not a huge fan of Matsumoto Drill Kenkyujo but his work is pretty detailed and cartoonish, and I enjoyed the art here. The characters are... extreme archetypes. I wouldn't call them very developed, but they don't really need to be considering the content.
I'd say whether you enjoy this series will be based almost entirely on whether you can tolerate or enjoy the extreme sexual aspects. It's just so prominent and ridiculously over-the-top that that's what's probably going to stick out the most.
Unfortunately, the series was a little too over-the-top and it apparently got the attention of the Japanese national police so it got canceled shortly after the first volume was released, leaving the series basically unfinished. However, the reboot of the series, Kono Yo o Hana ni Suru Tame ni, is actually apparently more of a sequel, so some characters here may show up there eventually. That series still has fairly over-the-top ecchi stuff (the volume includes in-depth descriptions of the color and shape of each characters' nipples) but it's way subdued when compared to this and probably won't run into legal trouble. It also manages to be interesting in spite of the lack of sexual content! So, if you want more of this story, I would recommend the reboot. As for the original, it's a bizarre anomaly and is worth checking out if you want to see just how far the content in manga can be pushed.
UPDATE: Well, well, it looks like the reboot has changed magazines and become much more obscene in more recent chapters. We shall see if it manages to finish.