I went to see UP this weekend and while I can't say that it's my favorite Pixar film it's still very enjoyable and the animation is astounding. I was probably a little disappointed that it went in the action/adventure direction it did so early in the film, but I guess it does so because that is one of the main themes of the movie. Also, I felt that it asks its audience to make several large leaps of faith, but then, it is a cartoon.
SPOILER WARNING!!!
I was kind of surprised by the level of violence at the climax: the film's villain (who didn't seem to care who was in his range of gunfire) was shockingly and assuredly dispatched for a human in a Pixar--not to mention Disney--movie (unless there are plans for a sequel afoot.)