I saw Avatar last night. As everyone and their mom has already said, HOLY CRAP the effects are spectacular. It's breathtaking in its otherworldly realism. And that would be enough to save it even if the plot sucked. The plot did not suck, fortunately.
It's a mashup of the plots of Pocahontas and Tarzan, set in space, so "original" isn't a word I would use to describe it. It bludgeons the audience over the head with a mile-high environmental Aesop, although it's partly justified beyond ideology by the science fiction (all life on the planet somehow forms a giant interconnected brain with a vague sort of sentience). Most of the characters are pretty two-dimensional (very ironic considering the effects), although the acting is very good.
The supporting characters were not fleshed out enough. The nerds are uniformly motivated by science and love of study and an academic respect for nature. The military dudes (that is, the ones who don't rebel and become tree-huggers) are tough-guy jocks who don't care what they're shooting so long as they're being paid to shoot something and are blindly loyal to their commanding officer. The mining executive is motivated by greed. The military folks who do rebel magically have stores of humanity that their more unquestioning comrades don't, but no reason is given for that. The only character that gets any real development is the main character, Jake Sully, whose character development is quite good.
I'm rather surprised that James Cameron couldn't deliver a believable love story in this film. That was the weakest link in the whole movie. Titanic, for all its detractors, was indeed an emotionally visceral (if historically unlikely) love story - that's no small part of why 8th grade girls went to see it 20 times. Avatar has the reverse problem - the star-crossed love story makes sense, but it failed to deliver any sort of credible emotion to me.
The love story aside, everything else about the plot was excellently executed. The plot may not be original but it may be one of the best tellings of this plot in film.
As I said at the beginning though, the effects could make up for all these shortcomings and more. "Effects" is really the wrong word to describe them though, because that implies that they're finishing visual touches. These are not finishing touches, these are the meat of the setting. This movie puts stuff on the screen that is so real that it requires no suspension of disbelief. The Pandoran flora and fauna is wildly imaginative and vibrantly realized; whatever creativity Cameron lacked in the plot he made up for in the incredible world where it all takes place. Hammerhead rhinoceroses, giant black panther-things, packs of wild greyhound-things, helicopter insects, jellyfish maple seeds, Quetzalcoatlus-like flying reptiles, six-legged zebra cavalry, trees with mute sentience, floating jungle mountains... all magnificent.
In any case, I can recommend the movie. It's definitely worth the price of admission.