I didn't get it.
First, it's all unreliable narrator and stream of consciousness. Those are fun tools to use but I would thank you not to use them to tell your entire story please. Some grounding in reality is essential for things like plot, even if there isn't much of one. It wasn't much fun when Faulkner did it and now it's not even fresh either.
Second, it's about the feeling and quality of dancing but it never lingers enough on the actual dancing for us to understand the Apollonian and Dionysian contrast the characters talk about. They're forever blabbing that exquisite technique is one thing and untrammeled feeling is another and the star needs both. They say the understudy is better at feeling and the star is perfect but icy. But the director never shows us the difference. It's a movie about dancing that doesn't give a fig about dancing and mostly skips over it.
Third, I don't like any of the characters. That isn't necessarily fatal for a movie unless said movie doesn't have anything else going for it. This movie doesn't have anything else going for it.
Fourth, the gross ballet dancer injuries were impressive but still gross.
Fifth, I already knew that pretty actresses are pedestrian without fancy makeup and lighting; you don't have to rub my face in it.