Black Rock Shooter it's a 50-minutes-OVA of the band Supercell, based on one of their songs (Black Rock Shooter).
The story it isn't very complicated and is a little predictable. It's pretty but sadly, at least for me, don't make tears flow.
The art is very good. Nice designs and animation, but there are moments where backgrounds feel a little dead, they are a bit static.
The music is great, they really made nice compositions. But the worst is the voices. It didn't seem to me that voices (or the acting of them) matched characters.
In general, it isn't bad at all, nor boring.
Since it's something not so long, I recommend you to see it.