When I first saw this in 2008 (the first series) it was the first TV drama I had ever seen which immediately made me buy the DVD so that I could watch it again.
The first ten minutes probably wouldn't hook American audiences, but as soon as the guilty verdict was passed I could feel my stomach dropping to the floor. It was the inevitability of getting dragged into the criminality of the prison system and the vulnerability of all those who aren't equipped to keep their heads above water by means of sheer brutality. I wasn't convinced by the authenticity of Con O'Neill's portrayal of a solicitor employed within the rather starchy UK justice system, but his performance was so utterly compelling that I'll forgive that minor deviation.
David Harewood was evil incarnate, yet chose understated, intelligent, brooding menace for his character, rather than what could have been a comic book portrayal of a prison kingpin. By the end of it I felt emotionally shattered and it's certainly put me off going to prison.
Seriously, this should be made compulsory viewing for wayward teenagers, although not for any who are awaiting their day in court.