It's worth seeing in 3D or 2D. In my opinion, however, it's not worth a second viewing in 2D. I definitely wouldn't buy it, I don't think the movie is that special. It's an amusement ride with a serviceable plot, taking the 3D away is like removing the loops from a good coaster.
Let's face it people, if Avatar had been released in 2D only and at 2D movie prices, it would have been profitable (along with cheaper to make) but no where near the current 2 billion worldwide total. Then again, if 3D technology hadn't been available, Cameron wouldn't have made the movie in the first place. The point of the movie was to immerse audiences in an alien world. I think that was Cameron's dream, just like it was Lucas' dream to make an nearly all-CG movie with the SW prequels. Cameron definitely made a better movie, but the movie services the technology and without it I think it's a bit of a hollow experience.
Objectively, the CGI in the movie isn't even that good... let me re-phrase that. It's quite excellent, but I don't feel like it couldn't have been made five years ago. It's the 3D capturing and rendering technology and the visual direction of the picture that really takes advantage of it to the fullest that made the movie special, and I think watching it in 2D is like listening to the symphony through one speaker on AM radio.