I saw it this afternoon. Six hours after the end of the show, I'm still (1) going over the details in my mind and (2) tearing up. I'm pretty sure I've never cried over a movie before, but "UP" isn't a movie; it's a myth. Don't think "Harry Potter"; think "Orpheus and Eurydice" or "Eros and Psyche."
In recommending that I see "UP," a friend mentioned a plot element, in a way that influenced my viewing of it. I don't want to offer any spoilers, and the whole thing is put together with such care and narrative economy that almost anything would be a spoiler. Just see it.
(And yes, it's worth seeing in 3D; I was afraid that, like so much of today's Hollywood product, it would be "about" the special effects rather than about anything in particular. That wasn't the case. The 3D was used to very good effect, but after the first few minutes it didn't occur to me to think "I'm watching a 3D movie for the first time in forty years" or "What an interesting effect that was!")
When I saw the first Godfather, I thought, "They're going to be teaching this in film school a century from now." Well, they're going to be teaching "UP" in literature classes five hundred years from now.