The problem with the movie is the same problem with the Star Wars prequel trilogy, primarily the complete lack of characters. And dialogue written by a 9 year old. The actual stucture of the movie isn't bad. If you're going to make a movie out of the first season, it's structure was the right way to go about it. The first two episodes, Aang finding his temple destroyed, the Blue Spirit, then the battle with the Northen Water Tribe. The special effects done with bending were also pretty decent. But that's all that was done right.
And neither of those things are what makes TLA great. Every actor is terrible, and all the characters are stoic and lifeless. The is only one character in the show that is the stoic one, and that's Mai. But she has a reason for being stoic, she's a stark contrast to Azula and Ty Lee so it becomes funny to have her reacting to high stakes with little emotion. But now imagine that every character was like that. Suddenly it's awkard to watch. Aang's fun and playfulness is replaced by a lifeless talking head.
Kitara's leadership and ambition is replaced by a talking head. Sokka's wise cracking, wanna-be playboy personality is replaced with an alpha male, over protective brother played by a talking head. Night was trying to add a dark, gritty element but left every other element out resulting in a boring movie that appeals to no one at all. It was a huge disservice to the franchise.
It had the potential to invite more people in the world and share the story. Possibly get a few people to watch the show it was based off and expand the fandom community as many of the new super hero movies are doing. But all it's done is embarass the fans and creators, now we have to consciously disassociate outselves from it and assure people who have only seen the film that it is the farthest thing from a good representation of what the show has to offer.
TL;DR You should only watch it so that you can better explain to non-Avatar fans why the movie is a bad representation of the show, and get them to watch the show.