We watched Monsters University a couple days ago and I have to say I was quite surprised. I had very low expectations at first after realizing it was a prequel during the first scene. In fact I almost walked away to do some chores. However, by the end of the movie I was impressed. It showed me the story telling potential of a prequel, which I had previously written off as an impossible task after the weak Star Wars movies as well as the horrific Assimov infill Foundation infill novels.
The movie did the basic job of a prequel, illuminating the characters and the relationships of the main protagonists. Unlike other prequels that seem to repeat the genre structure and tropes of the original, Monster’s U also does a good job of telling a different and amusing story. It’s not a groundbreaking story, but as a sequel to another children’s film it doesn’t need to.
Ultimately it made my wife and I want to rewatch Monster’s Inc, to see how knowing that original story would be different now that we know the backstory of these guys. I’m not sure if there’s a higher recommendation that I can give than that.