It starts as a interesting cop show and the first seasons deserve a change. As seasons go by the intro music goes from great to not so much - and the show follows suit.
It looses its way and try to be a champion of moral. Suddenly cops are not so interesting anymore and you are watching a social commentary about school children and corrupt politicians.
The show shall have that it's hard to place. They kill off people you are rooting for while most of the major villains go free. If you think this is a spoiler I can assure you will never guess what I'm saying.