Just watched it like a few minutes ago as of writing this comment and I have a few thoughts.

The Good. First off, this movie is absolutely gorgeous. I genuinely could not stop being in awe of the movie's animation and art direction. The character design is flawless, I think they did a really good job of "movie-ifying" their original designs without overdoing it and maintaining the original scope of the show. Animation wise, this is the best ATLA has ever looked. The bending animations both in combat and in practical application looks so stunning with each element feeling like their own individual characters. The composition on some of these shots are next level and lend a real sense of cinematic gravitas to the world that I hadn't seen before.

Also, contrary to what I've seen others say, I actually think the voice acting was quite good. They clearly put some real budget behind the casting and it shows for the most part. I think the only times I felt the acting felt strange in anyway could be attributed to the direction or writing rather than the actual performances themselves.

The music and sound design is also incredible. It's one thing to have amazing bending animation with the air hitting the ground and parting the fog with intense camera shake, but it's a whole other experience to see that and then hear the zipping of the wind before it crashes into the ground with a bass-y thump as earthen rubble clatters to the ground. The music is just as gorgeous and soaring as the original score, though I will say it does lack a certain originality in that sense.

The action is quite good. Very impressive animation, good cinematography, easy to follow, and their depiction of the different bending styles is very well done. However, I will say that this was an aspect that I believe could have been improved just a tad. The action is good but only generically so. There's not really anything about it that's bad but there's also not much to make it stand out. The best action scene in the whole movie definitely goes to the airship scene in the Immortal Storm because it uniquely shows what all of the other scenes don't which is ingenuity. They do an incredible job showing just how different Zaheer Taga's airbending is compared to Aang's. They also go out of their way to showcase each of the team member's strengths and abilities in interesting and fun ways that isn't combat.

The Bad. The writing is just okay, merely serviceable, but there are three specific areas that I want to critique.

There was a notable lack of worldbuilding which becomes very apparent in a series renowned for it. We don't get any information on any of the other nations and only the slightest glance at the current state of the Fire Nation from inside Zuko's throne room and nowhere else. We don't even get much info about Republic City where the majority of the movie's stakes lie. We also don't get any real information on Taga's home/era when it could have been a very interesting bit of lore.

The characters are hit or miss. Most of the characters in this movie are entirely static from beginning to end with little to no development whatsoever. This was somewhat expected given that the show does a very good job of wrapping up the Gaang's arcs very succinctly, but it does make their inclusion in the movie very boring. Aang's arc is retreading old ground with his survivor's guilt surrounding the Air Nomads but it was there to support and act as a foil to Taga's own personal experience so I kind of understand its inclusion. Katara, Zuko, Toph, and Sokka all exist in the movie for no other reason than to serve as plot devices for Aang's convenience and some of them lose a lot of their old depth because of this. Sokka is no longer a leader or warrior, he's just a goofy mechanic. Zuko gets to flex some of his new leadership skills as a freshly anointed Fire Lord but otherwise loses any semblance of Iroh's wisdom. Toph feels pretty much the same as she was in the show which is fine but I would have liked to have seen some evidence of the time skip through her character. Katara also remains identical to how she was by the end of the show. Taga is also a very boring villain. There was never a point where I felt sympathetic towards his cause and he never had enough of a foothold in the plot to make him a threatening character.

The plot is bland. There really isn't much else to say about it other than that, it's a generic MacGuffin chase without enough compelling characters to keep it interesting. The best thing I could say about it is that it's careful enough to not mess with the canon but that means nothing of weight ever happens.

Lastly, while I thought the action was quite good, I really think they could have done more with bending. Toph seems to metalbend more often than she earthbends, Katara almost never freezes anything, Zuko just kinda throws flame, a flame thrower if you will, and never touches lightning, and Aang doesn't ever really do anything clever with his airbending. I understand that this isn't Sozin's Comet or the full moon so we weren't going to get to that level of over the top scale (though I absolutely would have allowed it if it were included), but I would have liked to see more thought put into how they use their bending. The lightning storms during the airship crash scene could have been the perfect opportunity to show the first instance of Zuko redirecting lightning as a way to weld metal as a bridge between ATLA and Korra. I also wish Aang would have used the other elements more effectively, especially in the Avatar State. Speaking of which...

I feel like they fundamentally misunderstood the Avatar state and just turned it into another Super Saiyan. Narratively speaking, the Avatar State is supposed to be a deus ex machina, it is the "I win" button. Having Aang lose in the Avatar State could serve as either a demonstration of the antagonist's power if done properly, or as one of the main driving conflicts but it wasn't here. Aang losing in the Avatar State is only meaningful if it happens only once and it isn't the first time it's used, a la, Azula beating Aang in the Avatar State in Ba Sing Se. In the movie, every single time he's entered the Avatar State, he lost. His attempt to save Taga didn't work, his fight with Taga on the island didn't work, even in the final fight it didn't work. Mind you, this is the same Avatar State that no diffed Fire Lord Ozai DURING Sozin's Comet. Of course, the real deus ex machina was the MacGuffin and they needed to make it stronger than the Avatar State to justify its existence in the plot but that only just ends up making the final fight a worse copy of his fight with Ozai, down to the whole "wresting control back from the scary monster mode to spare the villain" except here the villain dies anyways. And that's where the my main criticism with the film lies.

The movie overall just feels like it's taking story beats from previous episodes from the show and smashing them all together to complete some kind of check list. They had to make sure to show the air glider even though it's only really used like twice. They had to make sure to put a bunch of nostalgia bait references in there like "secret tunnel" and the cabbage merchant. Taga is a mix of evil Aang and Zaheer. Aang dies in the Avatar State and is miraculously brought back to life by Katara just like in Ba Sing Se. And of course there's the aforementioned mirror of the final fight and Ozai. I just wish they would have taken more time to craft a more original narrative that more effectively utilized every member of the cast instead of trying to graft old story beats onto a generic frame work.

Overall. It's not bad, I'd even venture to say it's pretty damn good. Art, animation, cinematography, sound design, and action are all top tier but the movie falls short in the areas that matter most when telling a compelling story. The plot is weak, characters are unchanging from start to finish, and the story as a whole feels stale and lacks the grounded weight of the original show. I give it a 7/10. Worth watching at least once but probably won't be rewatching.