From what I've heard, this anime have caused some controversy. Still, it was the first anime I saw(officially), and has remarkably been on the top of my list since. First impressions are hard to beat.
The story really caught my eye. It is heavy, sad, dark and moving and contains a deep morale. It shows some pretty serious things about exclusion, prejudices, abuse, repressed memories and mental terror, which really sets the mood for the series, and keeps it in a serious aspect.
The animation is very good. It has good details where it's focused, though some moments are better animated than others. The colouring is very well and the characters are very nicely done. Especially the "vectors" and the dismemberments are very "nicely" animated. Good transitions and shading combined with great lighting.
The sound is pretty good. The voice actors did their job, and sound effects are pretty good too. But what really got me was the music. The classical piece "Lilium" was a very moving experience, as it is a sad, yet beautiful tune.
The characters are very lovable, well most of them. Others are easy to hate. The main characters you'll get to know, and empathy can easily grow, especially for Lucy/Nyu. You'll get to know their past and their personalities. You also get to know some of the secondary characters, as their story is intertwined with some of the main characters story.
Overall, this series left a big impression on me. Maybe as a first impression of anime, but the story really gripped me, and I wanted to cry for some of the characters. I really recommend this anime to anyone who wants a strong story with a deep morale, as it is the best I have seen.