666 Satan, written by Seishi Kishimoto (twin brother of Naruto's author Masashi Kishimoto), inspite of what it's title might make you think, is an tipical shounen series that tells the story of Jio Freed and Ruby Crescent and their quest to find legendary O-Parts ("magical" items left by an ancient civilization).
Even with this simple premise, 666 Satan has in fact a relatively complex plot, in this manga Seishi Kishimoto used various elements from religions all around the world, but most of all from Judaism (so, if you fell you might get offended or something, you shouldn't read this), to create a new world, that lives
an constant conflict, derived from it's own history, and to make it more beliavable, it's filled with a great variety of towns, each with it's unique wheater, culture and what not. In fact the story itself deserves an perfect score, however, the way it was told wasn't flawless. The pace wasn't always well conduced by the author, rushing at crucial points, and taking a long time to finish arcs that weren't really that important. Although this new "world" created on 666 Satan is really great, the plot itself is quite predictable and filled with clichés (especially towards the end), borrowing elements from many other classic shounens, so, if you already have some "luggage" on the genre, you will be able to tell where the story is going at some points, which makes the experience of reading this manga much less enjoyable.
Given the fact that the Kishimoto brothers started drawing manga together, it's kind of a given that they would have a similar style. This tells all about the art in 666 Satan, it's exactly like Naruto's: simple and clean most of the times, but detailed in the most exciting moments (it gets specially good when he is drawing demons); with lots of varied landscapes; and girls with unusual faces.
By reading this manga I got the impression that in the author's mind, all girls fall in love extremely fast, ALWAYS. There is not one female character in the show that is not in love with someone (usually someone she meet a few days ago). Also, all the "romance" in the manga seems rushed: in one chapter the characters don't care/hate each other, in the next one they are completly in love, which pass a feeling that there is a missing chapter in there. Inspite of this unusual trait in the female characters, they, overall, have an quite captivating personality, even if sometimes they tend to fall back in the tipical shounen character (the same can be said about the male characters).
One of the highest points in this manga (and what makes up for some of it's defects) is basically the fights: as there are countless types of O-Part's, all fights are different from one another, giving the impression that the stronger O-Part or OPT doens't necessarily will win, it will depend on ability of each one to think of the better strategy to use his O-Part and the battleground in his favor, this dynamic makes most (if not all) the fight very entertaining and exciting.
Summarizing, if Seishi Kishimoto had written this manga with a little more experince (perhaps with one or two manga already published) he could have turned 666 Satan into an great shounen (on the same level of Bleach or Naruto), but nevertheless, he wrote a very good story that, even if it's not an "must read", it's recommended for every fan of the genre.