Pixar worked hard to make sure their use of 3D cinematography in UP was properly integrated throughout their creative process so that it would be something that added to the movie experience and not a gimmick.
And when I saw it in 3D, I got so caught up in the movie that I completely forgot about the 3D-ness of it. Watching it again later in 2D, it really wasn't a significantly different movie.
The only movie I've ever seen where 3D added something to the experience was an IMAX documentary about fish played in a ten-storey high screen. Combine the three-dimensionality with the peripheral-view-filling nature of the IMAX screen and you really have something. I think it might work better for documentaries than for narratives, though.
