It's a pleasant (if overlong) anecdote on a subject which shouldn't be pleasant nor anecdotal. And the final, recent real-life scenes, while definitely disturbing, feel even too urgent and important compared to what we have seen before. One can't deny that the movie is well-made, at times very well edited, and directed by an experienced helmer with obviously a strong commitment to the theme. Yet, deep it isn't - intentionally, maybe.
In a nutshell, this was my experience with the film. Very clear from the onset that Spike Lee's mission is to portray these horrible people as hucksters, dopes, unserious people, all leading to an ending that effectively undercuts its own message by saying that these people are dangerous. The most charitable reading is that the idiot man-child played by Paul Walter Hauser (y'know, the guy who plays only idiot man-children) takes David Duke's message to a horrible new level and that that is the legacy of David Duke: that hatred is a thing that is passed down and becomes real. But I'm not sure I buy that as the lesson of the film. I think the better reading of Blackklansman is that Spike Lee wanted to have it both ways. He's good enough of a filmmaker that he doesn't really pull it off (although I do love the casting of Topher Grace the dorky David Duke), and yet the film still mostly works for being wildly un-suspenseful. I don't know what I was expecting but this is a Spike Joint. It is not Inside Man or even 25th Hour. It's a roaming free-range dialogue about race and politics. With one big exception, it's at its best when it winks at the present (such as that great "Re-elect Nixon" poster) and it's worst when it shouts (when a white cop tells Denzel Jr. that one day this country will elect a racist)... and that big exception is the scene with Harry Belafonte that is so powerful it renders the rest of the film effectively moot. When this scene began, I really didn't like what it was doing and it really took me out of the film. By the end of that horrifying story, I was certain nothing in the rest of the film was going to matter as much.
A mess but worth seeing. I prefer Sorry to Bother You, to which it is inexorably linked through white voice.