This movie was fantastic and is definitely worth a watch. The visuals were absolutely stunning, the score was spot on, and there were so many unbelievably imaginative things packed into this movie (including, but not limited to mechs, vehicals, space ships, animals, environments, and so much more).
One thing that other reviewers have commented on is how flat the story is. It's predictable and not very complicated. I do agree somewhat with this statement but I must say that despite this the story was executed very well.
One other issue people are taking with the story is why the humans came back to Pandora. Spoiler alert here. Humans did not return for whale brain slime. They came back to terraform the planet. The whaling ship thing was basically a side quest in the grand scheme of things. This movie is meant to set up a sequel and I think it does this beautifully and I hope we don't have to wait another 13 years for it.