I saw the midnight showing last night. As expected, the effects were amazing. However, the story was predictable (with some Bush bashing and a "man is evil" theme thrown in), and there were some lapses in logic (SPOILERS - the aliens "waking up" suddenly to the tractors, Jake not snapping Quaritch's neck while he's hanging right in front of him, etc.), but they were forgiveable. The acting was fine, but not stellar (Worthington is the tamest leading man I've ever seen).
Avatar is something to see for the experience (though I did find the 3D glasses a little bothersome). I love most of Cameron's films, but I guess I was expecting too much. Or maybe I was just too tired! I'll watch it again to find out. Overall, very enjoyable.