I am a fan of the animated series, so I will probably go and see it. But I have heard that there is a hell of a lot of exposition - I mean they basically took a 400 minute series (season) and tried to cram as much of the characters background, locations, mythology etc. into a 90 minute film. This is going to make it hard to watch for some people I am sure.
I will echo the worRAB of a poster previously, watch the animated series.
I feel this is another one of those movies I will have to explain why I am a fan of the series. Like Harry Potter, I am a fan of the books, but when people I know watched the films they wonder why I am a fan (the sixth film especially) and I end up defending myself along the lines of "the books are better honest". I feel this will be the case with Avatar: The Last Airbender too.