Just an anecdote:
I saw Toy Story 3 in 3D. A few weeks later, I saw Toy Story 3 in 2D. Obviously the movie had no problems with being in 3D, as they could easily render a second camera - no crappy upconversion. But there was nothing added to the movie by being in 3D either. It was a great film, but not even the descent into hell scene was particularly enhanced by the third dimension.