It's not my favourite Tarantino movie, but it's not my least favourite either - it's somewhere in the middle. I haven't seen Jackie Brown yet, but while I liked Once Upon A Time... over the Hateful Eight and Death Proof, I can't really put it over any of the others. (then again, he has a really good filmography in my eyes)
The first hour or so of the movie was a little slow, but it really picked up in the second half, with some scenes giving me vibes of the opening scene in his World War Two movie about Brad Pitt killin' Nazis that I can't name or else risk accusations of censor dodging. Unlike that movie or Django Unchained, I was unfamiliar with the context behind this movie, but it still did a great job with building up tension, and I knew that something terrible was going to happen. (or did happen in reality - I think a lot of people figured, from the announcement of this movie and the two Tarantino movies that I mentioned, that this movie was going to divert from historical events and end with the Manson "family" being massacred in a bloody fashion)
Margot Robbie was a little underused, and she didn't get much dialogue, but she still did a great job with the role, getting across her character's nerves, enthusiasm, and relief with acting alone. As well, she made for a great contrast with Leonardo DiCaprio's character, a rising star compared to his falling one. Speaking of DiCaprio, he REALLY showed off his range here, in a different way from Django Unchained. Thanks to his character's role as an actor, he got to show himself in all sorts of roles over the course of the movie, with a deep-seated insecurity and nervous stutter that I did not associate with Leo as an actor.
All in all, I thought the movie came together really well. I thought Mike Moh as Bruce Lee was wasted, though, and I really wish that the initial Wikipedia summary that I had seen (where Bruce Lee shows up to help beat up the Manson "family") was accurate. I chalk his big scene up to Cliff's unreliable memory, because the least realistic part of the movie (and that's including the flamethrower) was the idea that Brad Pitt could last more than five seconds against Bruce Lee in a fight.