I would have to say, it didn't live up to my expectations at all.
Well, here is my SPOILER review.
The first thing I have to say is the characters are freaking flat. None of them is likable. None of them makes me feel emotionally attached to. None of them is really fully developed.
We have Kouta, who had a dark childhood from watching his little sister and father being killed by Lucy / Nyu whom he considered as a friend. We have Lucy / Nyu, who as a child was bullied and betrayed by normal humans. We have Mayu, who was sexually abused by her stepfather. We have Kurama, who lost his newborn and wife, because his newborn was a Diclonius. I'm supposed to sympathize with these characters I guess. But they are so poorly-developed and so lacked of vibe. And the flashbacks didn't help either. At times I felt myself waiting for the emotional touch and it felt like it almost reach me, but then disappeared quickly.
What disturbs me most is, they didn't even try to explain why Lucy / Nyu was having split personality. I mean, she had been living her past few years in a lab facility and then suddenly when she escaped, she had split personality??? That's just too convenient for the plot, which brings me to the next point.
The anime opened with intriguing gory scenes of a Diclonius escaping from a experimental lab facility. Then it built up to pre-climax with explanations on events in the past, including the childhood of Lucy / Nyu and how she befriended Kouta. There were little hints at the first few episodes signalling something happened in the past, and then flashbacks towards the ending which supposed to disclose the "twist". Well in short, the big twist was - *drumroll* - that Lucy / Nyu as a child was the killer of Kouta's family. I think this is the most interesting part in the whole of the series.
The gory scenes were, well gory, and that's it. They didn't leave a strong impression on me and neither were they disturbing, particularly #35 (Mariko) who was Kurama's daughter. I felt that more could be done to this character. She was supposedly raised in a lab since she was born and isolated from all other human beings. Her becoming of a blood thirsty killer and her attachment to the idea of Kurama being a father were not well-explained and simply unconvincing.
And what was the experiments conducted in the lab facility anyway? What exactly were they trying to achieve? It didn't seem to be solely for academical purposes. Towards the ending it was shown the director of the lab, Kakuzawa took off his wig and revealed that he, too was a Diclonius. Quite a reveal but I needed to ask - and then what? And in the same scene we saw that Arakawa decided to hide the identity of Kouta, being related to Lucy / Nyu - again, for convenience of the plot.
The character of Bando seemed very redundant as well. He didn't make any significant contributions or advancements to the plot. And the part where Kurama fixed his artificial hand in exchange for him to kill Mariko - first of all, why did he think Bando can kill her when she is so-all-powerful? Second of all, how did he even know Bando would be at the beach?
I found myself asking "that's it?" towards the last scene - surprised to find that it concluded with a mere 13 episodes. I've always liked short anime but, this one just left you hanging. Its open ending is the worst in the history of open endings.
The last scene led us to believe that Lucy / Nyu survived and came back to Kouta. If so, how? The organization behind the experimental lab facility appeared to be having quite the influence with the government, the police and the special forces. It couldn't be this simple that they left her alone after all these hard work of capturing her alive.
All in all, Elfen Lied was a total disappointment and certainly overrated in my opinion.