I talked to some folks who did see the movie, and had not seen the animated series. They thought the movie was good.
I remember hearing one comment of, "People are expecting oscar level work all the time. If you can get out of that mindset, you'll like the film just fine".
But again, this person hasn't seen the cartoon.
I have yet to see it, and am in no hurry b/c I'm a fan of the animated series, and I know I'm going to be disappointed.
But having said that, this may get people who haven't seen the animated series to check it out.
I think there should be spinoffs. There should be a Katara spinoff called Bloodbender.