... A reasonable way to spend two hours (hint: it's a three hour movie).
... Implausible geography and improbable ecology provide fertile ground for demonstrating the state of the art in impressive CGI vistas, advanced image composition and motion capture. A demonstration of just how far all this has come, stitched together with bits recycled from classic SFnal stories.
... The paper thin stereotyped characterizations of the human population serve as a counterpoint to the depth of the visual artistry.
... A super-extended climactic battle scene seems de rigeur in modern epic cinema and Avatar honours its commitments here with vigour, ending with a resolution that begs the question: "what happens a week/month/year later when the next batch arrives?"
I am left with the impression that had I seen it in a 2D theatre I might have found it rather flat. (crickets)
As it was, it was an enjoyable ride that leaves few lasting impressions. Not bad for $300 million.
* - to wit: parallax problems on the periphery of the screen, a few eye-watering pans (mostly those involving on-screen computer displays), and all of the usual eyestrain problems you have when you play with depth of field in a 3D movie and have semi-interesting things in the background that people may want to (try to) focus on. Plus (of course) the seemingly irresistible temptation to continually make the point that THIS IS BIG, and HERE'S A(NOTHER) SWEEPING VISTA. That they seem to (mostly) hold their fascination through the extended period does Mr. Cameron credit.
(The first 3D movie I saw was Treasure of the Four Crowns. Things have come a fair ways since then.)
(And yes, I saw it in IMAX 3D)