I first saw this documentary in 1976. It took several days for me to shake the chilling, disturbing effect it had on me at the time. I just bought the DVD and watched about half of it. It still has the same effect, particularly if you have not seen it before.
What makes it so upsetting? It's not what you've come to expect in a documentary about Nazi Germany. This is the Nazis through their own eyes. The entire film is archival footage from the time. There is no narration. There are no post-facto talking heads. In short, there is no editorializing, and, lacking that, you start to see why the German people were willing to follow such evil people.
The bulk of the film is from before the war. The highlight is Eva Braun's home movies of Hitler and the Nazi leadership. We see Hitler relaxing at Obersalzburg, talking to friends, greeting children, petting dogs. As the film says at the beginning, it is necessary to see the human face of evil in order to recognize it the next time.