I posted about this in the main WDW parks board before I knew this thread was here, but I'm going to post here, too:
I had a real problem with the 3D looking murky/dim/losing details, like watching through sunglasses. The 2D version was vivid and gorgeous. I saw some mention of "Real-D 3D" vs "Disney Digital 3D" - is this the problem? I suspect my theatre has the "real-D" version, since the glasses for Up were the same as the ones they had for (the non-Disney) Coraline.
I loved the movie, thought it was funny, emotional, exciting, heartwarming, gorgeous, all the superlatives I've read in other reviews. This is the whole package: characters, story, animation, score... I'm not going to play "Rank the Pixar Films," especially since I'm still on my initial adrenalin rush from this, but I'm already wanting to go back and see it again.
And let me say, even though I'd seen a lot of clips, and felt like I knew a lot of spoilers, there was a lot I didn't know, and even the things I knew going in were better in context.