I hate to go against the grain (actually I lie) but I've seen one episode of this series, and the only reason I would possibly watch more is if my friend, who fanboys over the damn thing, forced me to at gunpoint, and geared me up in one of those things from A Clockwork Orange holding my eyes open and someone dropping water into my eyes to make sure they don't dry up.
In the first five minutes this girl, stark naked apart from a helmet, massacres a small team of elite, heavily armed and heavily armoured soldiers with INVISIBLE BLADED HANDS. It's implied she's been experimented on, it's understandable that she's a bit angry, but there's a stupid amount of blood and gore involved. I seem to remember a part where they fleshed out the life of one of the guards before he was decapitated in a pathetic attempt to make us feel sorry for him, but I'm not so sure about that part specifically. Anyway, she's shot in the head as she escapes, breaking the helmet and revealing her face, and she washes up onto a beach, stark naked.
When she's found, she now has the behaviour of a cat and the mental capacity of a three-year-old now that a large chunk of her brain is missing. And there are a multitude of ways the series' writers could show that someone is now mentally handicapped or infirm as a result of injury. They choose to have her running around with her hands between her legs for some reason, then she sits down on her knees, and proceeds to urinate on the floor. This bothers me for a variety of reasons, not least of which being that it's horribly distasteful to suggest that, because someone now has the demeanour of a child, they can't even use the toilet. If this was the case, I'm sure one of the other characters could've just made a comment about it to let the viewer know. Or even if this was impliedly the case, I'd rather it be the subject of some pervert's fanfiction than actually depicted for our viewing pleasure.
To top it off, this infirm, now handicapped pink-haired girl is seen nude more than once, and the scene in which the man who's tasked himself with looking after her is undressing her, and his friend (who happens to be a jealous girl) walks in at an inappropriate moment, and thinks he's taking advantage of her. In any other context, that could've been funny. Here, it was just horrible.
And the odd "word coming up" slide that they did in Evangelion. "VERNICHTUNG". It wasn't worth crap there, but at least it was actually relevant. It isn't worth crap here either, and has nothing to do with the scene it showed up during, with one of the characters walking through a back-alley in Japanese suburbs. Especially in German.
And towards the end of the episode, even more stupid amounds of gore as it's revealed that, despite missing a large chunk of her brain, this girl now has a fully-intelligent split personality who can make use of the INVISIBLE BLADED HANDS. Then reverts back to being mentally retarded suddenly.
I would watch the full series before making a judgment, but in general if the first episode of a series doesn't interest you, it's unlikely the rest of the series will. The first episode here thoroughly disgusted me, and I've seen a lot of the same stuff in bad 80s anime such as Violence Jack, Urotsukidoji, Running Bean, and a lot of other old-school Manga Entertainment publications. They, however, had the excuse that a lot of such content was a trend at the time in an attempt to have shock value and attract viewers, since anime wasn't as much of a big market in the west at the time. Anime's come a long way since then, and Elfen Lied from my initial impression does a lot of the same crap in an attempt to be deep and meaningful, but succeeds in being just as artistic as the above examples. Read: Not at all.
Edit: I just realised how big this rant was, but given how much I hated that one episode, I'm not sure I'm entirely surprised.