I watched this film as part of a university class on Spanish National Cinema and 30 minutes into the film, only a brave few were left in the room. It is hard to grasp it on first viewing, especially if you don't understand Spanish (because not everything, especially words beyond newspaper headlines, are translated in the subtitles) or know a bit about Spanish history. Another thing that makes it a challenge to concentrate all the way through is the lack of voice-overs - essentially the film is a montage of images and footage paired with popular songs and/or tainted with different colours.
But I found that watching it a second time(and subsequent times after) and doing some research on the events mentioned in the documentary, I really enjoyed the film because it really does do a good job in subtly criticising Francoist Spain (and getting away with it because it passed the censors and was only banned because it was a documentary), at the same time encapsulating a lot of the emotions that the people felt.
Plus it has some really catchy songs! A must-watch for the adventurous (and studious) but not really for those merely looking for some light entertainment.