I've only seen two episodes, but I can tell you already that I'm loving Dexter and should have listened sooner to the people who were recommending it to me. I almost hesitate to describe the premise, since it's absurd, and hearing about other people describe it is what made me skeptical.
But that said, what you have is a show about a sociopathic serial killer who also happens to be a forensic analyst for the Miami Police Department. But he's a sociopathic serial killer with a conscience (except not really -- he's a sociopath!) who only kills other killers. Woo!
It's pretty ridiculous, but so far at least it's pulled off extremely well to generate an alternatively funny/creepy/gross story that also includes a reasonably compelling mystery (another serial killer seems to be toying with our friend Dexter) plot thread running throughout the season, and all-in-all it's pretty great -- I like it more than anything I see running currently on television.