The Last Airbender is a great show, I've watched it all the way through several times. I agree, the first season is a bit "kids-show" and it's the second and third season that is really great.
Korra was pretty good too, but not as good as The Last Airbender. What made Korra great at first was Amon, that guy was just the coolest, most mysterious, most bad-ass character ever. Though how he turned out wasn't as cool as the rest of his stuff, but it was still solid stuff.
The movie sucked. I was excited and waited for almost two years and it just turned out slow, dull and just awful. Amateurish actors and awkward scripts and dialogue.