i think this is one of the best shounen out there. the mangaka can, if he want, drawn this out as much as possible because the possibility of hijinks of a princess of monsters is boundless, but he chose not to do so but stick to his plot. i must say at first some part are just mind boggling and weird to me, because this manga have several dimensions, time lines, multiple universes and such, and you, the reader, might not even notice it, but at the end of the story everything work out perfectly. this just show the thoroughness of the mangaka, and a second reading might even more enjoyable as you can notice various hints and clues scattered through the stories.
there are various characters in this story, but i must say none of their personality stand out or endear to me, thus I'm not emotionally invested to whatever happen to them. however, all of them have their own roles in the story, none of them are dead weight thus i'm happy that they execute their jobs perfectly. that is to say, to me the plot carry the manga, not the characters.
the art is not blindingly beautiful that you want to stare at it, i think none of the character have much memorable character design. the princess have very ordinary design, the ordinary blonde hair and goth dress of a princess of darkness. but they are pleasing, and perfect for the story.
this manga don't have unnecessary fanservice. there are some obligatory panty-shots, but the author seems to tacked on it because his editor told him so.
all in all, there are still oodles of mysteries still in this manga, for example what kind of vampire is rurie, what bridge is the spider god building, what will happen to the alien inside the glasses guy head, etc, etc. the world the mangaka had built are ripe for sequels, prequels - i hope the spin off can sate some of the hunger.