It's not going to be for everyone, but I personally love it.
Granted, it gets incredibly...
Uhm...
Dark?
I mean it's dark already, and the animation style should hint to that fact (Which is one of my favorite aspects of the series, an older, rebooted animation style. I really like the art, despite what some may say)
To be honest, it's the kind of series that doesn't make too much sense until you've watched a bit of it. I's slow, not like the typical shounen series that would air on Toonami (Uhm, DBZ and the 30 long battle scenes don't count, okay?)
Casshern IS kinda brooding, but considering the world is what it is, and people wanna eat him, it'd be hard to not be...