Did not suck.
Though in respect to anyone who doesn't want mild spoilers, I'll put the rest below.

Beautiful. That's the thought that kept reoccurring throughout the movie. Yes, there is a plot, and it's a very very old plot, being basically "Dances with Wolves" only on a different planet with different technologies. However, although every single moment of it was completely and utterly predictable, do you know, that didn't really matter?

To explain: I must have listened to the "Lost Chronicles" guitar solo in "The Xenon Codex" maybe fifty times or more. I know it by heart. I know where each note will come, how long it will last, exactly how it interlaces with the beat. So surely I'd find it the most boring piece of music to ever hear again?
Not so. Becauseeach second is familiar, I anticipate its perfection, my mind yearns for each tone to be exactly as it is, in its place, perfectly falling within the whole until you can lose yourself in it.

That's where Avatar scored. The story is as known as the palm of your own hand, but each part of it is perfectly placed and paced. It all clicks into its slots, exactly as it should, and it doesn't stuff anything up, it doesn't fumble, it avoids the pitfalls it could have jumped into. And because you know the plot, you can concentrate on looking at how beautiful this constructed moving piece of art is. The 3d is only one part of it, though it certainly helps. There is a glorious obsession, from the slight difference in the pace of a waterfall that falls on a lower-gravity world, to the startling weightless opening scene.

It's not utter perfection, but then I've never said that about any film, and I doubt I ever will. Parts of the planet's biology/ecology are highly questionable, and the ending is simplistically hopeful in a way which anyone who has any experience of human nature will automatically doubt. Yet it's the closest I've seen to a complete visualisation of another world. I saw people walking out of the movie with their faces shining, and I have to say, I stepped a little lighter myself. It wasn't the "earth-shattering film that will change cinema forever" it has been hyped as, but it was a few hours of beauty in the world.
I will be going to see it again while it's still out.