I finished the first season of Dexter last night. I give it a B-. Michael C. Hall, who plays Dexter, is AWESOME, but he's working with pretty lame material.
At some point, people (I use this term loosely, as I'm referring to the creatures who populate the film industry) imagined a righteous serial killer. He is brilliant in his own dark way, seeking to improve the world through his cruel lessons (Seven, Saw) or by killing worse people (Dexter). My problem with this storyline is that it's trying SO HARD to be controversial. Yawn.
The other problem with it is that serial killers are far from brilliant. They actually tend to be brain damaged. They aren't great plotters--they actually have insanely poor impulse control. And they don't have anything to teach the world. They're just assholes.