Mediocre. The protagonist is, the narrator and Celes insist, "sick and twisted to the very bottom of [his] soul", but if it weren't for the constant reminders, there'd be little to indicate this. The gimmick is that rather than jumping into a hot-blooded training montage and defeating the heroes with fisticuffs, Shinichi instead captures them and blackmails them into giving up. Maybe he's not a shining, golden hero, but his 'twisted' personality has been oversold.

This kind of story is like a mystery. It lives and dies on the protagonist doing something interesting and clever and believable, but without the reader being able to guess what in advance. Does it live or die? I could come down on either side.

On the plus side, tempting the hero over to the demon's side is an interesting enough plot, if not really original. On the minus side... well, it's all the usual suspects. I don't find Celes constantly berating Shinichi and accusing him of being a pervert to be either funny or titillating, whichever the author was going for. In general, the jokes don't land very well--Shinichi playing the tsukkomi doesn't amuse me.

If the sequels have more interesting plots, they could rescue this series, but I'm not in a hurry to find out.