As far as the documentary as a whole goes, it is very good because it achieves something that is of the utmost importance--it instills excitement and appreciation within the viewer for the feature. While I must admit that I like "Once Upon a Time", I also have always thought it over-long and in need of a better editing (shortening a few scenes and not cutting out subplots like they did on its American release). But, given the commentaries by so many learned people it makes me want to go back and reevaluate the film--perhaps they have a point. Additionally, lots of great background material is given as well as film clips and production stills. It's a shame that the director, Leone, died so young. They only included a couple short clips from hi and couldn't really let him speak for himself about this film or his thinking that went into the creative process.
If you love "Once Upon a Time in the West" or love films about film making or are a film student, then this multi-part documentary is for you. Well written, coherent and filled with insight and enthusiasm--see this DVD extra and not just the feature film.