Story: It's a great story but I was expecting a lot more. I loved how action packed this series was and I was never bored from watching. I liked how you get to see how a character transform but I wished it wasn't so quick to end. I just thought they would spend more time with the bigger problem of the series. I was disappointed by how fast pace the climax and ending was. It was based more on the main character's traning which is normal to most animes, but that usually bugs me.
Animation: I love the animation to this series. It's everything I expect. I like the use of vivid colors and the blue flames. I thought it was creative how they used blue flames to show that you were a descent of Satan. Most people think red, black or purple to describe Satan. I love the blue flames. The animation is great and I don't have much more to add.
Sound: I don't remember much of the soundtrack but the opening and ending songs. I love the second ending song. It is the most memorable song for me and I enjoy that song the most. In the series, no music really stand out to me so I was disappointed by that but the action of the show was fast paced enough that I forgot about it till now.
Characters: Focusing on the twin duo, great depth of characters. You get to know their true motives and their realtionship to Satan(Not just the being Satan's sons) and how they are affected by him. The only problem I have is that their friends don't get enough charcterization but it's good enough to make the show enjoyable. I wouldn't mind getting to know the other characters more but from what I saw, I guess I can say it's enough.
Overall: It's a great action packed anime. I did like how in this series they made Satan appear "Human" for once. He is still a bad and evil guy but he does have a soft side. It just shows that he has "human" emotions. There is always two sides to people. Overall, I think it's a fun anime to watch but I wouldn't buy this series as a DVD set.