Angst is a hell of a drug. A little of it can be extremely powerful, but too much and you end up mired in sticky adolescent self-pity. Black Rock Shooter is a show that has a lot of slick animation, gritty fight scenes, and weird vaguely science-fictional concepts, but in the end its emotional core is nothing more than that same angst overload, and it's all rather maudlin and over-dramatic.
To be fair, the creators seem to have had the best of intentions, and at least tried to do something interesting with a property that could have been a simple hot-girl-kicks-ass series. The execution just falls flat, however, and trying to figure out the series mythology is like walking upstream through a river of mud. There's a good show in here, but it's not this. It's usually pretty to look at, but ultimately Black Rock Shooter is an unfortunate failure, and together with the dire Guilty Crown represents the nadir of the once-revered noitaminA block.