Judging solely from the picture here on anidb, one would think that this would be a standard magical girl OVA with no real innovation or complexity. However, without a doubt, this is one of the strangest animes I've ever seen. In Alien Nine, incredibly cute, shoujo characters are juxtaposed against startling violence. The plot begins in standard fruitcake mode (girls must roll around on rollerskates to catch aliens), but deviates from this formula very quickly. I, for one, have no idea what happened in this anime. The plot becomes pretty much indecipherable at the end, and the ending credits have an event that is especially incoherant.
However, whatever the hell happened over the 4 episodes, I enjoyed it. The anime's mix of cutesy girls and flat out disturbing gore provided a surprisingly entertaining mix. Whether the events were actually disturbing or merely unintentionally hilarious, I never got bored during the course of the anime.
As far as animation and sound go, I was fairly satisfied with both. As mentioned before, very cutesy character designs are placed in direct contrast to the remarkably disturbing gore and violence. This juxtoposition works surprisingly well on a basic visual level. As for the music, what at first sounds like standard happy BGM slowly becomes more and more sinister as you realize the true intent of the anime. Though I'd never listen to the ost by itself, with the anime it works fairly well.
What prevents the anime from being REALLY good, however, are the characters. The best of the characters manage to be merely unmemorable and uninteresting. The main character, however, is ridiculously annoying. Call me crazy, but a school girl who cries about 3 times an episode can hardly be called "likeable." I expect that if the three main girls had been redesigned and expanded upon, I might have cared about them a little more. Ah well.
At any rate, the anime certainly isn't as bad as the average score seems to suggest. I think the reason for the abnormally low score lies in the fact that this anime is reaching the wrong target audience. People looking for strange, disturbing animes probably look at the cute picture on AniDB and shy away from watching it. Likewise, people looking for fruity shoujo crap download this and are sorely disappointed.
The bottom line is: if you're looking for disturbing, gorey, and downright WEIRD anime, you can do a lot worse than Alien Nine. However, if you're looking for a cute, mindless romp across some sort of magical fairy land, you'd be better off watching horse porn.