Plot was solid. Very sweet and funny. Loved the 3D effects. I recommend you do the 3D although it would still be a good movie to watch without it. It is not entirely dependent on the 3D like most 3D movies are.
The plot was definitely designed for kids but with enough undercurrents for the adults. Jokes and "awww" moments enough to keep it interesting. I definitely could see the comparison between the robots in this film and the early Disney film "the black hole" but that did not make them any less endearing (think Short Circuit's Johnny 5 as well.). The bleak future portrayed was pertinent to the times and really got my kids thinking. My kids are 7 and 11 and they enjoyed it thoroughly. Got the video game on the way home and they were able to explore the environment as the characters which got them thinking even more. Overall a very good thumbs up. Definitely a better movie than the last couple that Pixar put out.