Well, let's just say it's a premise that has some risque overtones. I personally now avoid these kinds of manga, but they did have their appeal for a while.
The stories' main characters are devils --- beautiful immortals in the classic girls-comics-style --- who always wind up helping the humans (whose souls they are actually supposed to be eating). Moreover, our Hero(ine) Astarot, a particular devil with long black hair and a knack for starving (because s/he always winds up helping people instead of eating their souls), is being chased by another devil (the cool-headed but amorous "Beelzebul[sic]") who wants to marry him/her. But because devils have no gender, "marriage between devils is a tough deal in which one becomes the slave of the other." Astarot has no intention of becoming Beelzebul's slave, and spends much of his time trying to get away. (Really, both Astarot and Beelzebul are more male than female). But because Astarot is a push-over and helps humans a lot, he is forever in debt to Beelzebul for extra energy. Beelzebul usually winds up helping Astarot out, though as the more level-headed and calculating of the two, he also usually gets a small return for his investment (if you can't figure this out, go read the manga...). However, Astarot frequently shrinks into a super-deformed tiny version of himself (from lack of energy), and usually manages to get away.
In the meantime, the Queen/King of the devils, "Lucifer," is p*ssed at the wayward two, and goes over to the human world to bring them back. S/he takes with her "Satanakia," and the two wind up helping almost as many people as Astarot and Beelzebul (in addition to getting mistaken for rock stars a lot). "Lucifer" spends a lot of her time drunk --- a side effect from sampling human foods --- and poor Satankia must look after her at those times ("My lord Lucifer--- !!! Not again!!!").
The stories themselves center around the lives of living mortals --- people --- as the devils observe and occasionally interfere with the humans they come across. Astarot and Beelzebul philosophize on what it means to be mortal or immortal (though said discussions are usually interrupted by Astarot fighting off the amorous Beelzebul).
There are 2 books currently published (they come out about once a year); they keep selling out, which is really annoying. But they're light hearted (with a serious side), and a great read (at one point Astarot and Beelzebul play a devil version of Nintendo). No sex, no violence.
In retrospect, I can't say I personally would recommend the topic any more, but at the time I did like this short, two-book saga.