This show has been around for a while, and I had basically written it off as just another forensic crime drama (a genre that doesn't appeal to me). But I kept hearing people recommend it, so finally rented it at the video store and watched it.
It is absolutely great.
The story covers a big drug investigation in Baltimore, following various characters (on both sides) with greater or lesser connection to the police team and drug syndicate. It feels incredibly realistic, and the production team are all people who ought to know (e.g. one of them was a homicide detective there). Every character is a plausible human - even those who seem one-sided at first all get a chance to show what else is going on in their lives later on. Most of the characters are not only plausible but interesting and sympathetic.
The caliber of acting, writing and direction is also very high indeed - maybe even flawless. I certainly never felt that something happening in the show made me think "What the hell? Why did they put that in?"
From a little internet research, it seems that the production team are very focused about what they want the show to do - essentially, to illustrate various sides of a city and show the good and bad stuff going on.