Don't look closely at the plausibility of the story, but suspend disbelief and enjoy the actors, the characters they play, and the gorgeous scenery. I've long been a fan of Sandra Bullock, but I've never really "watched" Ryan Reynolds in anything, and he was the perfect foil. And it was great to see Betty White in such a delightful role!
"Up" has the best characteristics of a Disney film - plot that sparks the imagination, beautiful animation work (seeing the balloons come through the clouds is breathtaking), a villain who gets what he deserves, and the sense that real life has some sadness in it, even though it all turns out fine in the end. Plus there are plenty of laughs for kids and adults along the way.