I watched this for the first time last night, after a year or two of hearing people rave over it.
Honestly it's one of the best movies to come out recently, but don't be surprised if it's over-hyped. Sometimes expectations are so high, that's there is no way any movie can live up to it.
The story is pretty good, which basically is Europeans vs Native Americans, but in a futuristic alien setting. It's also one of the few movies to basically get away from dissing the military without backlash.
Granted the special effects are the best thing about Avatar. Will it hold up 50 years from now? Who knows, but I enjoyed it quite a lot.