My 16 yo son saw Avatar the first weekend and said it was the best movie he has ever seen. At the time I said "Really?!" LOL he was right. We went as a family on Christmas. I cried through the whole second half... it was so wonderful and awful at the same time. The science of it was wonderful, the scenery was breathtaking, the acting was fantastic, the action was thrilling and the social messages were compelling without being preachy.
I also found it to be one of those pg-13 movies which were totally acceptable for the younger set (allowing for the fact that I had to go out 4 times for potty breaks, lol)