Very good but not great taken purely as a film, its main weakness being characters that are a little too thin and rely overly on preconceived stereotypes - i.e. it's good for what it is, but it's no masterpiece like say 2001.
Technically though, it is a marvel. Pandora is amazing to behold and the 3D is the first I've found worthwhile - i.e. not a gimmick. I'm glad it's doing good numbers, as it is in no way a bad movie - Transforms 2 is my idea of a bad movie - but hope that Cameron will take more narrative risks in any sequels, and focus on a more original story.
8.5/10 overall for me. Very good, with the visuals and solid if unoriginal story covering over the cracks in the dialogue.
Try to be reasonable... its easier than you think...