I'm in two minds about Casshern. I loved the design, but some of the story points were just too random for me to cope with. That castle in the mountains? Where did that come from? Why did they suddenly happen upon it? What was that whole ten-minute section actually about? While some of the scenes are good for dramatic effect, they seemed to have little to no bearing on the plot whatsoever; it's like it was being pulled out of the air just to fill in inconvenient gaps.
But I have only watched halfway through; I was also rather ill and tired when I tried sitting through it. I'm not that optimistic that an OVA will be a great deal more productive with fleshing out and explaining the story, but it'd be interesting to see. If a friend of mine buys it, I'll watch their copy. But I doubt I'll get it for myself.