As one of the few people here (maybe the only person, I haven't seen anyone else admit this), who saw the movie before the show, I must be honest, if you've never seen the show and have no idea how good the show is, then the movie is good, but it will make you want to watch the show and after you watch the show, you'll look back at the movie and go "dahell was that s***? (turn back to the show) Man this is way better! The movie so didn't do this awesomeness justice." Then you watch it again, and you're like, "WTFITS?"
There was a mountain of epicness and detail lost in translation. Sorry. There just was. I mean the concept is good, but an hour or two is simply not enough time to cover everything that needs covering to do ATLA justice. They should have done two movies a season, and maybe done a little bit more to make the movie characters look and act like the show characters.
Also some personal gripes, drill helmets, lack of some of the shows most important and recurring supporting characters (Suki, Jet, Haru and Bumi), the guy who played Iroh looked like he should have been Roku and he lost his funniness, and I don't recall them even mentioning Roku or that Sozin's Comet was coming back around. Also the whole "They told me the avatar can't have a family" is EPIC FAILURE. That completely puts the kibosh on half of LOK as well as the whole FNRF redemption good vs evil internal struggle story line which was ONE OF THE MOST MAJOR COMPONENTS OF THE STORY.
Or at least that was my take on it. You're welcome to think what you like.