There were SO many bad things about this manga, but somehow, it managed to pull off a fairly entertaining story nonetheless. I was quite honestly surprised... and no, it's not one that relies solely on big boobs. I suppose if you were really into those the entertainment factor would skyrocket, though.
So what's bad about it?
Well, like other people have mentioned, there's occasionally a huge disconnect between chapters and scenes. Sometimes you just won't have a clue where you are, when in the timeline it is, and what on earth anyone is doing. It's jarring.
Secondly, the Anime Catholicism is somehow strangely offensive even if you come in prepared for it. I'm more or less used to awful Japanese misinterpretations of Catholicism by now... but when I first ran across them I was like THIS MAKES NO SENSE! WHAT IS THIS?!, and this is seriously one of the weirder ones.
Furthermore, the author doesn't do a good job of the setting right at the beginning. He never really explains the structure of the Adepts and... and the other people... in a simple fashion. Wikipedia is crucial in understanding the plot, and even then it seems like things happen just so they can happen. There's a prophecy from a random book, a bunch of meaningless but cool-looking religious symbolism, a lot of Russian that is probably madly hilarious to anyone who speaks Russian... you get the picture.
Finally, as for the ecchi factor.. holy crap it was wildly disturbing. That's not to say it wasn't well-drawn, but Wildly. Disturbing. Tentacle. Things. They are there. They abound.
But for all that it somehow managed to make a good story. It was very focused, for one thing. It meandered off into side characters now and again, but for the most part it focused on really establishing a solid core group of main characters who people could sympathize with. I particularly liked Sasha and how the author occasionally lampshaded his tsundere tendencies, lol. I still have no clue what on earth he's fighting for though.
It has sprinkles of humour throughout, and the battle scenes are pretty decent and easy to follow. Mostly. The romance/character development between Sasha and Mafuyu is far more adorable and much less depraved than what you'd expect for a manga with this much sex, and was a major plus point (if you discount that Sasha is just barely legal... in Japan. Which is saying something.) The author does a decent job with recognizable characters, and doesn't fall into the Loads and Loads of Characters trope... much.
Overall it's worth a try if you're looking to kill time with a battle manga and are not easily offended.
Or if you're really looking hard for drawings of people breast-feeding, for some reason.