I had a big discussion about this on Facebook recently.
Now, from my own point-of-view; I've never seen the cartoon series, and heard the film had been slagged off by many critics., so I watched with an air of dissapointment as well as total lack of knowledge.
And when i finished watching, I really enjoyed it. But - and this is the big but that some of my friends failed to see understand - it was because I had no preconceptions about the film.
From what I understand, the film is basically the first 13 episodes, but highly condensed.
As a first time viewer I had no problem understanding the concept of bending, or of the plot. There were a few questions that didn't seem to be answered in the film, but they were fairly minor ones (like why did it take 5 Earth benders to move a small rock, while one could create a huge create wave of earth?)
But other than that, I did enjoy it.
I have since tried to watch the cartoon and found it really unwatchable. It's nothing like the people in the film are.