It's a great film, a breath of fresh air for big blockbuster filmmaking. In the age of quips, and an excessive need to look at the audience and nod to confirm that "yes, we know big concepts are silly"... this has the same level of maturity to the way it approaches scenes and characters as great indie dramas, just with a budget and scope that makes it look bigger than the biggest blockbusters.
I was very high on it to start, after a little while it started petering out even though it remained very good, but then something would happen that brought it up again, and kept moving along very well. I do not know how a general audience is gonna react to the pacing of it, or the somber tone. It works very well for me, but I am definitely gonna have to be picky which ones among my close friends I recommend it to. It would be easier to recommend to people, had the story actually ended here.
The main point of contention is definitely going to be where it ends. I think what makes the ending feel jarring to people is what big blockbusters have conditioned us to expect from the third act. You tend to end a blockbuster with the biggest action scene occupying the third act, but Dune is a pretty extreme version of doing the opposite. The lack of a huge climactic sequence probably doesn't clue some people in that the end of the movie is about to come. I would say that to me the movie felt about as complete as Fellowship, which does have a grander third act (though compared to pretty much any setpiece in the following two films, it's pretty damn small and insignificant, just dudes in the woods), but it also could not make it more clear that there is absolutely more story to come. What'll also set people off with Dune is not even knowing if there's actually gonna be more to come, and if there is, when?
If Dune Part 2 (and maybe even a Part 3) is comparable to this, then collectively Dune has a real shot at being my favorite film trilogy (or duology), that would truly make this the cinematic thunderclap that one tweet claimed. Dune Part 1 just can't do it alone. Though as far as comparing it to other first-entries in a franchise? This is the best in a long time.