I think this OAV is definetely insteresting to watch if you already know the story. So watch it after reading the manga or even watching the TV series.
If you watch if before doing that you won't understand what's going and who the characters are. Plus, it could spoil some events of the tv series.
But anyway, the animation is pretty cool, the characters are interesting...
It is a story with supernatural events and it the kind of story that I personnally, like.
So I will go further in the TV series and come back finish this OVA later...