I actually didn't mind the movie. It stayed failry true to the series albeit removing much of the unimportant filler episodes.
I am actually a fan of M. Night for the reason that he is at least trying some original stuff different from the typical action movies you see all over the theaters. I liked The Sixth Sense, Signs, Unbreakable, and even The Village. Lady in the Water was ok, just a bit too wierd for my taste and The happening was just bad. But I digress...
I would give Avatar The Last Airbender an B-. It wasn't the best action fantasy movie I've seen but it certainly wasn't the worst.