They pretty much are and always will be a gimmick. They were a gimmick in the 50s, a gimmick in the 80s. A gimmick in the 90s. And now that pretty much every major live action franchise blockbuster movie is going to be in 3D, a gimmick in the 2010s. Avatar opened the door, and just because one film had massive success with it doesn't mean every film will.
Part of the appeal of Avatar's 3D effects was that the world was so colorful and vibrant, and that you wanted to explore, be a part of, and absorb every sense of it. A majority of the films being announced as 3D are simply piggy-backing on the success of Avatar - very few offer a world that you want to explore. However, that's not to say there won't be a few good ones.
The biggest concern is simply that 3D will become so mainstream so quickly that it will lose it's appeal. That, or everyone will own their own pair of 3D glasses, and ticket prices (and profits) will go down, that Hollywood will need to find a new gimmick (scents?).
In any case, studios shouldn't expect to have anywhere near the success that Avatar had, and (BIG GUESS) by Summer 2012, 3D won't be nearly the spectacle it was with Avatar in December of 2009.
