While not the most amazing movie ever, Avatar in IMAX 3D was definately worth the time and expense. The special effects are, indeed, as astounding as the hype would lead you to believe; the characters are enjoyable; and the plot, if not terribly clever, is engaging and has a satisfactory ending.
I'd strongly reccomend it, but definately see it in the theater (preferably IMAX 3D, its worth the extra $); it won't be the same experience on DVD.