I originally watched it with some slightly younger relatives that knew nothing about the Manson family stuff, so a lot of the parts of the movie that had an impact on me didn't have the same effect on them. When the hippie girl tells Cliff she's headed to the Spahn movie ranch, I said "Oh shit!", and they all just looked confused by my reaction.

Recently, I've been watching through all of the Tarantino movies with my son, and before we watched Once Upon a Time, I told him to take 15 minutes to read the wikipedia article about the Manson family murders.