Story: The story was absoulutely amazing. It was heart-breakingly sad at some points, but that was only because I was so connected to the characters. The story flowed well and although I was confused at some points, my questions were answered as I kept reading. Also, I was impressed with the amount of details woven throughout the story. It obviously took the mangaka some thinking to come up with this tragic yet wonderful story. Elfen Lied made me think a lot, and although it's a supernatural tale, I drew numorous parallels to the real world.
Art: I didn't like the art all too much... I can't really describe it, but the characters looked odd at some points. However I did like how the mangaka drew Lucy's alternate personalities so you could tell them apart (especially the eyes). Also take note that the art is very excplicit, so if you don't have a strong stomach or are offended by nudity, then I would suggest you not read this manga. Overall, the art was alright. I liked it at some points but not others.
Characters: I loved the characters! At first most characters were easily hateable, but as you learned more about their past, you came to understand them and love them. Nana was probably my favorite character because of her internal strength to protect her friends and family. She was probably the character most easy to sympathize with, too. Also, the characters were very human, even if they were exagerated, because of their emotions.
Overall I definitely recommend this manga. It is one of my favorites :)