After watching the first episode I thought "Wow, this is going to be epic". Sadly I was wrong. To summarize Elfen Lied: Fan-service (naked girls with kitty ears), melodrama and gore. The author probably thought he could have something decent mixing the three things.
The story: There is a "Diclonius species", they are born to kill and are humans, but they have cat ears. Yes, Cat ears. They were supposed to be horns. But guess what? Fan-service.
The story started pretty well with a girl killing some humans and escaping a military/scientific facility. She has "two personalities" or amnesia (whatever). The rest of the show is pretty much bullshit, some melodrama, then more gore and a terrible ending.
Would not recommend this to anyone. Enjoyment 1/10. Overall 1/10.