I saw it last night with my little guy and a friend of his. They sat in the very first row, directly under the screen so I'm not sure how well the 3D glasses worked there. But for me down front and all the way to the side (we came in late because I couldn't get them to eat their pizza slices faster), the 3D worked perfectly.
In fact, I couldn't stop myself from saying "cool" three or four times during the film and the short before it (about storks delivering babies, also in 3D).
The story is totally contrived and a mix of children's stories you've read or seen before (though the romance at the heart of it is a little more like golden age Hollywood). But what made it worthwhile for me was the art. It's computer generated Pixar imagery, but the 3D effect on beautifully rendered depictions of storybook illustrations made it seem like a genuine work of art.
Any of you with kids, or who remember your own childhood illustrated story books, know what I mean about how some of them are simply works of art you can't help digging for the drawings. In the case of UP computer generated, but still drawing-like in their stylistic elements and flourishes.
I highly recommend the 3D version (the glasses were no bother either, much sturdier and worked perfectly compared to what I remember from earlier Hollywood 3D experiments). Approach it like a museum or gallery visit, maybe a children's museum or gallery featuring children's book illustrations. In other words, just go to dig the visuals, and the humor and story and voiceover acting (which is excellent, led by Ed Asner as the grumpy old guy) will be extra reasons for your enjoyment.