The really interesting thing about BB for me is how they've turned Walter into such an unlikeable character. For most of the first two seasons I was solidly in his corner - sure, he was an anti-hero who did terrible things, but he did them for a righteous reason. In the last 6-8 episodes though, he has become a thoroughly unlikeable character who I can't sympathise with any more. Not only unlikeable, but irrationally angry about everything, and annoyingly incompetent (he was always that, but when I was on his side his incompetence was humorous, not frustrating).
It's a big risk to make your main guy totally unsympathetic, but they've done it cleverly by bringing in Saul as a clown, and spreading the story more broadly over the ensemble, which is fascinating from a writing POV.
What are some other shows that have transformed their main character like this? Off the top of my head, I can't think of any.