Pasting from my profile notes.

!!!SPOILER WARNING!!!

I am a sucker for stories like these. The trope of the main protagonist of a series being presumed dead or being wiped from existence is one of my favourite tropes ever, as we get to see how the other characters of the series are affected by their disappearance. What makes it work is that we are already familiar with the characters and their relationships, and we feel the same grief as the characters do when the protagonist is suddenly gone from their lives. This movie falls completely flat because it fails to live up to those standards.
Steins;Gate falls apart at the root when Okabe is not present. There is not a single likeable character in the series beside him, and for that reason I couldn't care less about them feeling sad. I have no sympathy or empathy for them, I don't care about their happiness, and I definitely don't care about their sadness. Kurisu is one of the most hateable characters in anime history, and watching her take up the role of protagonist for this movie made me hate what could in concept have been a good story. The worst part of it all is that the Kurisu in this movie is not the Kurisu that we followed throughout the original show. She has no memories of her time spent with Okabe, so all her feelings in this film feel superficial to the core. Why would she care so much about someone that she has barely interacted with before? Some might say that "oh, their relationship defies the laws of time and space", but that is just a convoluted way of saying that the writing is terrible.
Not only is the movie not entertaining, but it is completely pointless. We learn nothing new, and the movie actually ends up hurting the series canon. Kurisu being the person to inspire Okabe to save Mayuri and take up the mantle of Hououin Kyouma is frustrating, as it takes a lot away from what made that scene so impactful in the original show.
The first hour of the film is fine enough, as we see Okabe struggling with keeping his memories of all the worldlines intact without them overwhelming him. There is still the annoying and cringey humor from the show that isn't funny in the slighest, and Kurisu acting like a total tsundere bitch for no reason, but at least there is nothing game-breaking or offensive to the original show. It's only in the last half hour of the film where Kurisu takes over as the protagonist to save Okabe that the film takes a total nose-dive. There is also no structure in the logic of the film, as it's constantly shifting all over the place from idea to idea. In the show, we watched Okabe suffer greatly to achieve his goals, while Kurisu doesn't really have to struggle at all. She has some scenes of "oh no, Okabe is gone, whatever shall I do", and that one scene where she watched Okabe get killed because of her, but there is nothing there to make the personality changes she goes through feel validated and realistic whatsoever. Another logical fallacy in the story is the inner workings of the Reading-Steiner. The explanation of how it works keeps changing all the time, as we suddenly see characters remembering Okabe despite him "not existing", only for them to forget once again in the next scene, and so on.
The only saving grace of this movie are the scenes where Okabe is coming to terms with the fact that he will disappear, as we get to see the fruit of his development from the original show. He has learned so much over the course of the series, and it's refreshing to see him being opposed to time travel, compared to before when he was constantly eager to travel back himself again and again. I would have liked to see more of that, instead of what we got with Kurisu.
Overall, I consider this movie as an insult to the original series, as others too have said before me. There are some decent scenes in the first hour of the film, but nothing that makes it a worthy watch. I would recommend everyone to stay as far away from this movie as possible if you value your positive impressions of the original series. If Kurisu is your favourite character for some unfathomable reason, you might find some enjoyment in it, but if you are anything like me (or if you didn't even like the show to begin with), there is nothing for you here. 3/10

TL;DR

"Steins;Gate The Movie – Load Region of Déjà Vu" is an insult to the original Steins;Gate series. It offers nothing but logical fallacies and inconsistent characterisations, and the story is messed up from the core. It does luckily offer some decent Okabe scenes scattered throughout, but in general it is a complete trainwreck. I recommend the film for people who hate themselves and want to suffer, but not to those that have some form of dignity left in their hearts. Instead of watching the whole thing, I would recommend to just watch some clips that might seem interesting on YouTube, as that will save you the time and effort to slog through this disaster. 3/10