The first thing to understand is that coming to this film is not going to suddenly cast a huge amount of light on the main film. If you are still trying to (pointlessly in my opinion) work out why the star was reborn when the tree had died then you are a) missing the point, b) not going to find those answers here and c) missing the bl00dy point. However what the film does do is follow the creative process in six or seven chapters in a way that mostly adds value to the film.
By this I mean specifically the aspects of the production that add value to the themes within the narrative such as the lighting, the set design and the use of the macro photography for the space effects. These are explained and discussed in ways that made me understand the importance of them and the relevance within the narrative. That said I did think that too much of the hour was taken up with scenes of the scenes being shot quite interesting perhaps but these could easily have been cut in half without any detrimental effect on the film.
What is missing is also worth noting. At one point Aronofsky does make a joke about Batman and that only further highlighted that there is almost no gossip, rumour or coverage of the difficulties. In fact the issue over the film being shut down and the set sold off is dealt with in a single title card with no more detail than I have just given. Certainly the casting issues are not on the table for review or discussion. This is probably quite a lot to do with not burning bridges but it did make the film feel a little like issues were being skirted. Perhaps if it had focused intensely on just the creative aspect and not the practicals then this would not have mattered so much, but by touching on the challenges but not doing the major, perhaps infamous ones, then you feel the gap.
Despite this though I still found it interesting as an insight into the very creative cast and crew behind this film. A good third of it could have been edited down (or replaced with some of the more interesting practical challenges) but even still it is a solid documentary.