I think it's great and not necessarily a gimmick. I think it'll be tricky as lots of cinemas can't actually play the movie at 48fps.
This coupled with 3D will mean that the movie(s) will hopefully be a cinematic treat: http://thehobbitmovie.co.uk/the-hobbit-movie-hobbit-video-blog-4.html
This coupled with 3D will mean that the movie(s) will hopefully be a cinematic treat: http://thehobbitmovie.co.uk/the-hobbit-movie-hobbit-video-blog-4.html