Before I start, let me say that I don't watch anime that often at all - I only found this movie purely by chance (I got a YouTube ad and, by pure luck, I didn't choose to click away immediately). And I am so freaking glad that I did.
Now, I acknowledge that this movie has its flaws - every creation does. The plot is a bit much for the film to handle (it probably could've/should've been a bit longer, it might've given them a chance to do a bit more worldbuilding), and the ending could've tied things up a bit more than it did (like, what happened to certain characters after the climax? It would've been nice to know a few more specifics). But the appeal of this movie shines through despite these flaws.
First of all, the soundtrack is absolutely stellar. Several of the songs have made their way into my regular rotation, and I like to put the whole album on play sometimes when I need a playlist to listen to. I watched the English dub of this film, but both the English and Japanese versions of the songs are amazing. If for whatever reason you decide not to watch this movie, please at least listen to the music.
Secondly, the animation is some of the best I've seen. It's a bit reminiscent of Spider-Man: Into the Spider-Verse, in that it finds a balance between 2D and 3D animation. Some of the backgrounds in U are absolutely gorgeous, and I almost wish I was able to go there and explore it myself.
Third, even though the plot isn't as strong as it could be in some places, the sheer amount of emotion in some scenes is incredible. I started tearing up twice during this movie, and I can't think of many other films where I've come close to crying even once. And I think I was grinning like an absolute lunatic at the climax, because I could feel all the joy radiating off of the characters in the scene. And the ending, while again maybe failing to tie up everything neatly, had me feeling so happy and hopeful as I left the theater. It really had me believing in the potential for good with the internet and in the general goodness of people.
Overall, this film is one that I would 100% recommend. I left the theater wishing that it didn't have to end and already wanting to watch it again. I'm planning on getting the rest of my family to watch it as soon as I can. I tend to get a better read on a film and how much I really like it the second time I watch it, so of course there's a chance that my opinion of this movie may decrease when I do. But if the amount of love I had for it when I first saw it is any indication, I could like it a hundred times less and still think it was amazing. I simultaneously want a sequel because I want the story to continue, and don't want a sequel because I'm afraid it could never live up to the original. But if you ask me, that's the best way to end a movie, because it gives fans the chance to imagine their own endings, and makes the story seem more open and and full of possibilities.
In short, I loved this movie, I plan to watch it again, and I definitely think you should too.
If you've read this far, thank you for caring enough to read my opinion, and I hope this helped you decide whether to watch the film and/or help you form your own opinion on it. I can only hope that you enjoy(ed) it as much as I did!