My review! No spoilers...
Visually its simply stunning. Aside from the 3D its fantastically inventive and stylish. Some people are complaining that Pandora looks like a "Yes" album cover brought to life, and the Mechwarrior suits have been done a few times now, but they've still never been done quite like this. Lord of the Rings was famous for making Middle Earth real, and also for Gollum as the first fully realised Digital character. In this case I'd say Avatar has outdone it in some ways. There's no question that the Navi are more consistently realistic than Gollum, but they have the advantage of being more fantastical creatures, so they avoid the "uncanny valley" effect. Pandora is a completely realised Ecosystem, but again, because its fantastical, they get away with more. There are some shots however that are just amazing.
Then there's the 3D. You've never seen anything like it. Its not the "stuff flying out of the screen" effects. Its more a sense of depth. You feel like you're looking into a room, rather than at it. Even the computer screens in the Comm room are 3D , so you're seeing 3D displays in a 3D image. When you go into the forest, insects fly between you and the scene, you feel like pushing aside ferns to see whats happening behind.
The story itself is a cross between a Vietnam war movie and Dances with Wolves, but it really works.