It's a fairy tale of what could have been, if, starting with the familiar "Once upon a time..." Knowing what really happened, made it easy for one to root for the suffering of the evil and vile Manson family.
The movie drug some in the beginning, spending entirely too much time setting up the washed up actor and his BS backstory. I think they just wanted to make the part big enough for DiCaprio.
Pitt was really the star of the film and he made it work, and, like he said in the interview at the beginning of the film, he really did carry DiCaprio's load.