Avatar is great entertainment. I've seen it twice, once in RealD-3D and once in IMAX 3D. The latter is better. I've not seen the movie "flat." Both 3D technologies are worth the extra money.
If you've seen the previews, you know the basic plot. With Titanic, we knew the boat sank. Knowing doesn't ruin it. James Cameron is a good story teller. There's some stuff that's not explained in the previews that makes the implementation of the avatars more interesting, and locals (Nav'i, I think) are an evolved culture that is closely tied to their "animal" nature, the living spirit in all things, and their ancestors.
As with Titanic, the special effects are awesome, but are used in the context of an actual story. The story of of love is nothing new, nor is corporate imperialism. How it's all put together is convincing and engaging.
Are there flaws? Sure, but I'd be hard pressed to identify them in my wave of enthusiasm for the stuff done right.