I'd seen parts of the movie on the small screen, wanted to finally see the whole thing, and there's no better place to watch an epic scale movie than in a theater.

3D was ok. I could take it or leave it. At least in this movie it wasn't overdone; apparently Cameron kept many of the wide shots pretty flat to avoid making the ship look like a model. I guess it's a wash: how much the added dimensionality enhances the experience was balanced by the times when it's just a distraction.