Story-wise it's serviceable and fine. You'll notice ideas taken from many other movies. But as far as the "experience" I have never seen anything like it in my 39 years of watching movies.
I saw it on an IMAX 3D screen and after the first 20 minutes my eyes "adjusted" (I'd never seen an IMAX 3D movie before) and the "real" world ceased to exist and there was only Pandora. I really don't know what else to say other than "SEE IT!!!!"