I never saw The Last Airbender, or it's cartoon counterpart thingywhatever, but I heard that the movie was god-awful. I did see Avatar, though, and I actually thought it was fantastic. It was very long, yes. But.. I don't even care, I thought it was totally worth it. I think it was the first movie I saw in 3D, and though I generally don't find 3D "necessary" for anything, I really thought it added to the film. The visual effects were simply amazing, IMO. I think the plot is great, and.. Overall I really, really liked the film.
I thought so too, right up until the commercials were released. The sequel is in the works, but I have no desire to see it at all. It will probably be derivitive of the Pocahontas direct-to-video sequel XD The first Avatar was basically FernGully, but I don't think the sequel will be be based on it's direct-to-video sequel, that'd be sad.