Avatar in IMAX 3D is visually stunning. Once your eyes adjust to the 3D and its (still) inherent stobe effect, you're completely transported into this fantastic alien world.
The performance capture technology reproduces practically every nuance of the actors' facial expressions and body language, so the CG characters are completely convincing, and their alien world is blended seamlessly with human characters and live action.
The action sequences are indeed thrilling, and the storyline is compelling enough. The movie sometimes crossed the line into cornball territory. But it was still one of the best cinematic experiences I've had in a long time.