Hi Angie,
I'll be honest, there are some parts that are pretty darned intense. I took my 3-D glasses off as did my son (seven years old) for those brief periods. It seemed like lots of people had to kind of shake it off for a second or two - just here and there. Most 3-D movies that I've seen seem to lose their 3-D-ness at some point. NOT Avatar. It is pretty much on it's game throughout the entire movie.
Be excited but be prepared. If you are prone to motion sickness it might be amplified by the "big-ness" of the experience. My husband wanted to see it in IMAX and we were warned against it since that is even more intense- I can't even imagine what IMAX would be like. I was blown away by the "Real 3-D" effects and felt the movie was TOTALLY worth the experience!! We are going back again very soon to see it again.
Have fun!!
Pam