Well, I went and spent nearly 3 hours on this, so let's get to what I think
First, I'll start with the good. I enjoyed it better than I was expecting, but my expectations were rather low going in. But I didn't absolutely hate it. Also, this is undoubtedly the greatest display of special effects I have ever seen. Visually speaking, this film is second to none and is worth watching for that alone.
Now, to the less than good....this movie is nothing new in the way of a story and it doesn't do it nearly as well as one would expect. For a film that had a budget of half a billion dollars and was 15 years in the making, it was in a word, disappointing. Your classic good versus evil where the roles were so cliched that it had me rolling my eyes more than once at the characters/dialog during the film. Cameron's not-so-subtle jabs at corporate America and the military were downright disgusting (IMO). For a man that seems to hate everything capitalism stands for, this film has made him very rich....because of capitalism. Cameron's mantra seems to be borrowed from Doc Holiday in Tombstone: "It appears my hypocrisy knows no bounds."
Putting aside his in-your-face political overtones throughout the movie (and it is chock full of them, from denigrating anyone that is in the armed forces to Cameron's distaste of the war in Iraq and so on and so on) and getting down to the base of the plot, it is an average story at best. I compare it to the girl that shows up at a party wearing too much make-up and clothing that is too expensive for the event; all glitz and no substance. Other than the effects, there is not one memorable thing about this movie. One needs to ask themselves this; if this had been a direct-to-video film with a fraction of the SFX budgets, would you have thought it was anything great?
I'm actually glad that I saw it because it isn't the worst movie I have ever seen, but I definitely don't ever see the need to watch it again.
That of course is my initial feeling right after watching. I'm sure more will come to me as it sinks in.