the OVA are disapointing compared to the series.
A waste of time. Except for the fourth OVA (The Epilog), it's a simplification of the series (which wasn't very complicated anyway).
The fourth OVA is more interesting, it comes after the series and advances the story a bit. We even see, at least, one of the high powers of Kuruda do something else apart talking and acting superior : Scarface can fight. Of course he does that without moving, he's Scarface after all. That's great.
Irony aside, the last OVA only is worth watching. The origin of the shadow skills is even revealed and is very original. If it was possible to note specifically that OAV, I would give it more points.