At first glance, Alien 9 appears to be a cute little anime about chasing aliens on roller blades. Thinking this was the case, I bought it. I thought that they must have been exaggerating or in complete error when I saw the 13+ rating. Not. Despite its cheery and cutesy character design, this is a very dark and disturbing anime.
I should have known something was up when I kept getting bad feelings for no reason. Things happened in the anime that made zero sense, and yet I felt completely horrible while and after watching it. When I finished the series (4 30-minute episodes), I couldn't watch anime for the rest of the week. I felt dirty, like I had done something very wrong. Although the anime was violent and disturbing, it didn't seem enough to affect me for days. Then, I discovered that Alien 9 is riddled with symbolism - and some of it not so hidden. To call it dark would be an understatement. There are blatant sexual instances represented, expressions of deep and black pain, and perverted thought processes. I would never recommend this to anyone. I have a feeling that someone who has experienced firsthand some of the horrible things the symbolism depicts would pick up on the innuendoes instantly.
Last, but not least, there is some very light yuri (girl's love) tone in the anime - and it's more obvious in the manga.
All in all, Alien 9 is, in short, to be avoided like the plague.