We saw Up over the weekend, and I enjoyed it very much. As usual with Pixar movies, it's got some complicated emotions. The first 10 minutes of the movie is a summary of his entire life, and puts you right at the center of his love for his wife.
Visually, the movie is beautiful, especially the attention to detail in the nostalgic items from Karl's past. They've made the mundane objects he's attached to works of art. The snapshots especially caught my eye. They have textures characteristic of their time, and the imperfections typical of snapshots which somehow make them more evocative and realistic.
On the downside, the plot is absurd, even for a kids movie, almost absurd enough to get in the way of enjoying the movie. Some will find the emotional resolution of the story simplistic, but you have to remember this is still a rated-PG kids' movie.
The usual Pixar traditions are in place, including easter eggs, with a glimpse of an upcoming character, and an appearance of the Luxo Jr. ball. Here's a trivia factoid I haven't read anywhere, but I think may be true: this is the first Pixar movie with blood in it.
I really appreciated that the movie works without playing down to children with the usual poop jokes and kids sassing stupid adults. There are no up-to-the-moment culture references or pop music dance sequences. The biggest name celebrity voice is Ed Asner. The movie succeeds not because of gimmicks but because of solid story-telling.