If you were dying to spend another 3.5 hours in the world of Avatar, your prayers are answered. I will at least admit that, despite the predictability and lack of imagination in the plot, it only feels like about 3 hours. My biggest gripe with Avatar is that there is so much possibility and so little of anything new in it, other than the technology to create the film. And, yeah, that's something.
My only suggestion... if you are going to watch this, rewatch what came before first. I had a heck of a time remembering a lot of the salient points after the 4 year gap between films. And the script does a horrible job of reminding you of what went on before to get us to this starting point.
The movie could have been saved with a bit more focus on the acting. And, surprisingly, a bit more effort on some of the animation. There were definitely some scenes with issues in syncing voice and clumsier movement. But, overall, it is a gorgeously created and engineered world. And while the world is intended as a character, this third installment backs off that aspect more than the previous two in many ways even though it's at the center of the story.
If you were hoping for the humans being less stupid this release, you'll be disappointed. Apparently not only do we learn nothing from Earth, we actively attempt to repeat it. Mind you, that could work if humans were well established on many planets, such that they look at some planets as just raw material (regardless of the consequences to the indigenous life). But that isn't the universe of Avatar. Or, if it is, it hasn't been made very clear yet. But the level of stupidity is beyond credulity.
So go for the visuals, squint through the plot. There is plenty of action and some minor plot advances. Will 4 and 5 ever be made? Probably, but I've given up trying to predict an industry that is changing by the hour. My life would not be less for this being the last of the series, but it would slow the technical advances in the industry. Remember, Abyss wasn't such a great movie (even the original cut, though that was better), but it did produce Morph, which forever changed gaming, movies, and animation. Such events have their purposes.
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