I personally thought it was amazing. I felt the story (although predictable yes) was still strong and compelling enough to really draw you into the world of Pandora.
The special effects were second to none, absolutely outstanding, The People looked so graceful and real too which really added to the experience.
The direction was also good, James Cameron made a good choice to make the movie long enough to complete the story as is, rather than taking shortcuts that you could see other film directors taking.
IMO, one of the best movies of 2009, second only to Tarantino's Public Enemy.
I did not see it in 3D unfortunately, but will be taking a trip to the nearest cinema in Peterborough soon so that I can experience this apparently 'revolutionary' new technology.
In the works (of James Camerons mind) for around 17 years if I remember correctly, and you can see the depth that he came up with from that amount of time researching. A new language, an entire new ecosystem with genuine scientific depth, compelling relationships and complex moral undertones.
Sorry if this sounded a little high brow or long-winded or whatever, I just felt like it deserved some well thought out praise
I greatly recommend you see this movie while it's at the cinema if you have not already, I can't imagine the experience being quite so awe-some on a 19" monitor screen with a 2.1 speaker set tbh.