__Spoilers for the original anime, no spoilers for the movie.
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Steins;Gate The Movie is a heartfelt send off to the "main timeline" in the original series. It tries its best to put a neat little bow on the series, and mostly, it succeeds. Where it doesn't succeed is when it defies established canon, such as bending and changing time travel mechanics or having characters act a bit heightened for the sake of the scene. The latter is a very small gripe and only happens in the beginning; the best example of this is Kurisu acting a lot more emotional from the get-go. Along the same lines, there were a couple of moments where characters just conveniently had certain objects on their person at a time or place where it didn't feel natural for them to have it - nothing too important for plot, usually just something to facilitate a cute moment. There are various scenes like this that are obvious nods to fans, and while I did find them pretty sweet, they did take me out of the story just a bit. Very small complaint for me, but your mileage may vary.

Now, the former might be a larger point of contention for fans: this movie did change up the mechanics from the show quite a bit. It's hard to discuss it without spoiling things, but this movie takes some concepts from the show and bends them to serve this new story. Specifically, the retention of vague memories across world lines (déjà vu) is expanded to accommodate the new main conflict, and other rules are changed or broken to support this as the new focal mechanic. Remember how in the anime, there could only be one active world line? Well, that gets throw out the window. Remember how the S;G world line was meant to be free of time travel? Yeah, well, out the window. Time leap limit? Divergence? Bah, who needs those? Out the window! Basically what I'm saying is that you're gonna have to go into this with the acceptance that it's not canon. Once you get past that though, you are in for a treat, because this is a love letter to Steins;Gate through and through.

The premise alone is quite good: Okabe struggles with derealization and can't seem accept the Steins;Gate world line as reality because of his conflicting memories from other world lines, causing him to disappear from reality. (Yes, I know that makes little sense according to the original mechanics, but trust me, just go with it.) Kurisu, much like Okabe in the original story, must use time travel to make him recognize the current world line as real so that he can exist in it again. That's pretty much all I can say without spoiling the plot. Things progress somewhat similarly to the anime, but with a focus on Kurisu. This allows us a more intimate look into her character than the series had time for, all while being faithful to her characterization there. Sure, I think some of her reactions were a little exaggerated, especially some comedic moments in the beginning, but they do fall in line with how she has acted at different points in the series. It was also just really nice to get some more of Kurisu x Okabe. While I think the series did their relationship justice, it was nice to indulge in something more focused on their romance, complete with callbacks that will make any fan smile from ear to ear. I don't think this would've fit well into the series, so I'm glad to see it here.

And that is more or less where I land with this movie - I don't think a lot of these things would fit organically into the series, but I'll be damned if I didn't enjoy the hell out of them here. The focus on romance, the fan nods, and even the sci-fi concepts--when viewed independently--were extremely enjoyable. If you are able to see this as an independent story from the main series, or can reconcile your headcanon, I think you'll enjoy it immensely. But if you are the kind of person that can't get past the changes, you are safe to skip this entirely. Much like the "Egoistic Poriomania" OVA, this is a non-canon addition for fans who want more, but it doesn't necessarily add much to the series. The series ended in a very satisfying manner, so some might also want to stay away to preserve their sense of finality. That said, I would definitely recommend this movie. It's a very solid movie on its own, but it is also a nice cherry on top if watched after the main series. This gets a solid 9/10 from me, only losing a point due to the breaking of existing rules.