Monsters Vs. Aliens is a great for people who enjoy pretty pictures and find a plot with new ideas confusing. There is about every Cliché shot and line of dialogue possible in this movie. That said, If you're watching it in 3D the movie is very visually stimulating and the render quality is stunning. Also, there is a lot of 70's and 80's pop culture references so if you're over thirty years old you might actually get some of the jokes.
In this movie Susan, an anime looking character, is going to get married but instead is hit with a radioactive meteor which, for some reason, turns her into a two-hundred-foot tall, super strong, behemoth. This surprises just about everyone except for the U.S. government which quickly locks her away with several other creatures deemed “monsters”. Meanwhile, a Hitler-esque alien named Galaxar wants the radioactive substance that collided with Susan for some undefined reason and thus sends a giant in impregnable robot to retrieve it. Upon discovering that they cannot blow up this robot with conventional weapons, the U.S. Government then decides to send in Susan and the other monsters to defeat the robot.
My recommendation is if you have kids; rent it when comes out on DVD/Blu-Ray. If you want to see a movie at the theater in 3D I would suggest Up set to come out later this month.