The series is pretty good when it focuses on the GSK cases and efforts to track him down. Unfortunately, it's interrupted far too often by a competing narrative about the author of the book on which the series is based. Given how much time was dedicated to both, the title should be something more like, "I'll Be Gone In The Dark: The Golden State Killer Crimes and the Life Story of Michelle McNamara."
Also, I think things could've been wrapped up in 4 or 5 parts. By the time I got to parts 6 and 7 I was thinking, "how in the world are they going to get in another two hours of content about this?"