First off, this is a great show for those who appreciate complex, interesting characters. To me, the characters and their journeys are what this show is truly about. Dexter, the closeted, vigilante serial killer who happens to also work for the Miami PD as a blood spatter analyst, is one of the most twisted yet likable characters on TV right now. (Never thought I'd actually root for a serial killer.)
Second, there's a reason that this is a cable series. It does have a significant amount of violence, hard language and other adult content. I would not recommend this show for anyone under 17.
Third, if you happen to have Showtime On Demand and haven't seen the series before, its worth going back and watching it from the first episode. (If not, the first season is on DVD a would be good rental for a rainy weekend.) The writers did a great job of slowly revealing the history of Dexter to compliment the main plot line within the first season. I had several, OMG moments while watching due to the creative twists.
All in all, this is a great series for adults who aren't too squimish or easily offended and can appreciate thoughtful character development.