'Like all Pixar films, the new one seems both impossible when you first hear of it and inevitable once you’ve seen it. The neurological basis of emotions turns out to be a surprisingly good subject for an animated cartoon, the brightly personified emotions almost like mini-animators themselves, pulling levers and pushing buttons to retain control of Riley’s reactions as she negotiates the moody, unruly emotions of pre-adolescence, as joy and Sadness attempt to overcome their differences and return to base. It’s as if Docter were asking: what animates us? Some of his inventions are happier than others. The Islands of Personality that mark the outer perimeter of Riley’s internal landscape — Friendship Island, Family Island, Hockey Island, Goofball Island — had a slightly anodyne, theme-park feel to them, and I was relieved to see them crumbling and slipping into the abyss, as Riley grows more sullen and withdrawn, like those cliffs tumbling into the sea at the end of Inception: M C Escher meets The Beano... There is a transluscency to the film's ingenuity: you can see the idea glowing beneath the skin of the movie, which at times resembles a live brainstorming session of the Pixar braintrust. What keeps it from the ever-present threat of abstraction and allows it to find deeper anchor is the increasing size of the role Docter gives sadness. I mean that literally: the actual size of the actual role given actual Sadness. Initially bossed around by the others — “I’m not actually sure what she does,” says Joy “I’ve checked” — Sadness comes into her own as they meet up with some of the discarded relics of Riley’s childhood, including her old imaginary friend, Bing Bong, a pink elephant-like creature with cat-like whiskers who is destined for the memory dump and who taps into exactly the same melancholy thoughts of their inevitable obsolescence that drove the toys in Toy Story.'
— from my review of Inside Out for Intelligent Life