I have only recently watched a few 3D movies, and I find them to be a bit of a hype, not to mention the higher cost associated to watching these in cinema. What I mean by hype is that there are really only a few moments in the movie when the 3D is significant, but otherwise, it's mostly close to 2D. Anyhow, if I ever get the opportunity to watch Avatar in 3D, I would because I have heard rave reviews about it. It was probably one of the first ones to come out in 3D.
I haven't been to too many 3Dmovies, and I don't know if it's worth paying the extra money I guess it depends on what sort of movies they are, when I used to take my son on the school hollidays to the movies he wasn't to know whether thwy were 3Dor not.