This film has some flaws - hyperbole, some moments of B-movie-ness and overstating the importance or relevance of events in the 1930's, but other than that, it's amazing.
It is very well put-together - it presents all the inconsistencies of the case in an easy- to-follow, fluid narrative, all while poking holes in the official story from every angle.
It is very well researched and has the benefit of time and hindsight to reveal things that were unknown, covered up, delayed or not presented into evidence.
It was not released in the UK, and the recommended "87 cuts" (that it would need to be certified for release in the UK) would've made it into an incoherent music video rather than a documentary.
If you're a conspiracy theorist - watch it. This is just amazing.
If you are not a conspiracy theorist but don't always believe everything the government says (Richard Dearlove, head of the British Secret Intelligence Service: "We have never killed anybody in the last 50 years"), it's a great watch because it's filled with shady stuff.
If you believe every single word the government says and have never doubted anything and will never doubt anything, this film is a great work of fiction. It's a fun story to watch.
All in all, highly entertaining.