Entertaining space adventure on a planet far far away with a very different civilization and weird plants and animals.
The action scenes are really cool and a pleasure to watch. Also the humanoid hippies called the Na'vi and the animals look incredibly life like and you kinda believe that they are real living beings.
The problem with the characters is that they are either good or very evil. Nothing in between. Also the story is very simple and you had predicted almost every little step way before it happened.
The 3D looks quite alright except for a few problems with distances (e.g. when Jake looks up from below the waterfall and the creature is way too close.) But to be honest, I could live perfectly happy without 3D movies for the rest of my life. It's a lot of hype, more than a thing you really need. I'd prefer a bloody great story any day.
All in all very much an experience and an adventure from an action and CGI point of view. Just don't expect that you'll have to think too much.