The plot of this manga is quite funny and interesting, Rion have some close moments with each one of the guys, there is a chance to learn just a little bit about her past and the marriage it's not the only surprise that is waiting for her. It's not really hard to guess what will happen next but just watching her struggle against her father, three 'husbands to be', and seeing like she treats all the men as the most evil creatures in the world...can be quite absorbing.

The art is good, not something really eye-catching, but no bigger complaints on that matter.

Characters. Quite simple, nothing special, it would be hard to get attached to them, because readers don't really get a chance to know them better
but they are quite easy to be liked/hated and i think it's better than thinking of them as indifferent.

The weakest part of this manga is the ending.
It just stopped in the middle of action, as if author is letting the readers solve and imagine what happened next on their own, leaving many questions without an answer.

The Devil Within, it's a short, simple manga that one can read while having some spare time.
It's not a 'must' but it won't hurt trying - unless u really hate when the plot ends in the middle of the story.
Personally I think that it could become something greater only if the story goes on.