Let's sum up the reviews as "other-worldly," which could mean a lot of things but in this case is very good (mostly). From Manohla Dargis in the NYT:
With "Avatar" James Cameron has turned one man's dream of the movies into a trippy joy ride about the end of life -- our moviegoing life included -- as we know it. Several decades in the dreaming and more than four years in the actual making, the movie is a song to the natural world that was largely produced with software, an Emersonian exploration of the invisible world of the spirit filled with Cameronian rock 'em, sock 'em pulpy action. Created to conquer hearts, minds, history books and box-office records, the movie -- one of the most expensive in history, the jungle drums thump -- is glorious and goofy and blissfully deranged.
From Joe Morgenstern in the WSJ
This is a new way of coming to your senses--put those 3-D glasses on your face and you come to a sense of delight that quickly gives way to a sense of astonishment. The planetary high doesn't last. The closer the story comes to a lumbering parable of colonialist aggression in the jungles of an extragalactic Vietnam, the more the enchantment fizzles. Much of the time, though, you're transfixed by the beauty of a spectacle that seems all of a piece. Special effects have been abolished, in effect, since the whole thing is so special.
From Kenneth Turan in the LAT:
...To see "Avatar" is to feel like you understand filmmaking in three dimensions for the first time. In Cameron's hands, 3-D is not the forced gimmick it's often been, but a way to create an alternate reality and insert us so completely and seamlessly into it that we feel like we've actually been there, not watched it on a screen. If taking pleasure in spectacle and adventure is one of the reasons you go to the movies, this is something you won't want to miss.
Among the top critics on Rotten Tomatoes, "Avatar" gets 95 percent. So, what about the box office? Gitesh Pandya at Box Office Guru is looking for around $75 million for the opening weekend, which seems a bit low given all the hoopla ("I Am Legend" scored $77.2 million, making it the biggest December release so far). Certainly, the buzz is strong: Fandango reports that the film accounted for 87 percent of its weekly ticket sales (Bloomberg). It is worth noting that for all the adulation, "Avatar" is not considered Best Picture material, at least based on the national and regional critics associations. In other words, a technological feat that's light on story.