The glory of Pixar's films as that they touched on universal themes, all the way through, that just happened to also tickle kids. This one starts out that way, and then settles comfortably into being a kids film.
The pacing is perfect, the animation looks terrific, the emotional range is wide, and there are a few sly jokes you might not catch the first time around. That's okay; you'll want to see it a second time anyway.
You never know what you're going to get with this sort of thing, if people will sit on their laurels and bask in the concept or go to the effort of working out a compelling story. Luckily the folks at Pixar aren't known for taking the easy way out.
Adventure is about escaping death's closure, but it's also about clinging to life's openness - letting one escapade's end evolve into something new. "Up" is a tremendously affecting story of indomitable love, infectious spirit and embraced possibilities.