Saw it last night. I can't quite cosign on Walter Chaw's rave, but if I lost a best friend to suicide a couple weeks ago, I have no idea how a movie about any sort of close friendship (even a toxic one such as this) would hit me.
At the same time, I don't agree at all with saying Robbie's Sharon Tate lacks personality or is just a dumb blonde. Despite substantially less screen time, I thought she had just as much depth as Rick and Cliff, and she provides the film its soul with her longing for fame while knowing it's temporary. She was not only my favorite character here, but one of my favorites in Tarantino's entire filmography. The scene when she watches herself and the joy she takes from the audience was one of my two favorites in the film (the other being the filming of the TV pilot, where DiCaprio's bad acting is hilariously perfect for the character). I'd say Robbie should be a strong contender for supporting actress awards.
If it hadn't been for the sheer viciousness and brutality of the final act, which is shocking and jarring to a degree that I'm not sure has any dramatic justification, Once Upon a Time would have easily been my favorite Tarantino film. It's a lament for an era that's gone and the good aspects of that era, while simultaneously acknowledging all the toxic elements of that era that led to its inevitable demise.
I really like the points Alissa Wilkinson makes in this essay on the movie's ending:
I took it for granted Cliff most likely did kill his wife, which may have also been a lament over Natalie Wood's early death - that was the first thing I thought of with that subplot. Let's also not forget one of his first lines after the opening TV interview is a racist insult of Mexicans. I thought the point of both was to establish he's an antihero and to expose the racism and sexism that plagued (plagues) Hollywood and necessitates a reevaluation and deconstruction of many films from that era, which Tarantino clearly knows, as can be seen in Bounty Law and the bad TV pilot being just as much a critiques of Westerns as they are tributes.
One early line states "Roman Polanski is the hottest director in Hollywood right now." That line only makes sense and has any greater meaning if we know why in six years from the time of the movie that will very much no longer be the case.
Tarantino's premise seemed to me to be: what if evils like that had never happened? Which puts this in a slightly different category of revisionism than the revenge fantasies of Django Unchained and Inglourious Basterds. There'd unquestionably be other evils that still have to be dealt with, and the film shows that, but dreaming for one night of a world with four fewer murders because of two drunk, stoned antiheroes makes this film less conceited than I often find Tarantino's films.