The consistency that Pixar has displayed in the quality of their movies is nothing short of amazing. Starting with Toy Story, they've raised the bar, either in the story or in the animation or sometimes, both, with each new film. Up keeps their track record intact. With fully-fleshed out characters, a simple but mature story and fantastic animation, the film is another winner from the Pixar stable.
Aside from the fact that it is a cartoon, there is very little in Up that could make us categorize it as a children's film. From its protagonist(Frederickson, a 78-year-old man) to its story(bonding between the old, lonely man and a young kid, Russell) to the emotions it explores, it is a rather mature film. There are also very few physical, slapstick jokes and no talking animals. There's no doubt that kids will be entertained by the bright colors and the interesting action sequences but calling Up a kids' film will be a disservice to Pixar's high aspirations.
The emotions evoked by the sequence that details Frederickson's life with Ellie rivals those evoked by any live-action film. As the two meet, fall in love, get married and grow old together, they become the kind of 3-dimensional characters that most movies strive to create but never succeed. And Up achieves this in the span of 15 minutes! The sequence also explains why Frederickson has become the cranky old man he is. Up creates the perfect foil for Frederickson in Russell. Exuberant and compassionate, he is really funny too. And this is important because his character is what makes Frederickson's transformation believable.
The film's middle segment almost looks like Pixar wants to make it up to the kids for ignoring them so far. With talking dogs(though these are technically dogs with electronic collars), a colorful bird, a nasty villain and a couple of high-energy action sequences, the emotional quotient during these portions is not much. But it is likely that it is this segment that will have the kids most excited.
Considering that Up is a Pixar film, the top-notch quality of the animation is a given but it is still impossible not be awed by what unfolds on screen. The multitude of expressions on the characters' faces, the bright but natural colors of the things around them, the detailed rendering of the surroundings - all these are impeccable and impressive as always. But its the smaller things, like a character's faint reflection seen in a window or the scattered rainbow formed in the mist at the base of a rainbow, which stand as testimony to the attention paid to details, that really take our breath away.