Emily and I went to see Monsters vs Aliens the other night. As an animated children's movie I thought it was ok. It's very silly and makes some off the wall jokes. The story's pretty predictable and easy to follow. It's no Pixar movie but it does provide an enjoyable time.
We didn't see it in 3D, but there were some scenes that were in there solely to take advantage of the 3D technology (a paddle ball coming towards you and things like that). Emily doesn't get any extra enjoyment out of 3D movies and they cost a bit more than their 2D counterparts.
Emily couldn't get over the fact that the main character's head (the 49 foot woman) was proportionally larger than her body and no one else had the same affliction. I chalked it up to the animators trying to make her look "cute." My favorite parts were the parts that included Stephen Colbert playing as the president personally.
All in all, if you're a big 3D fan then go see it for that; but purely for the movie, I'd say it's worth renting if you have kids, but you're not missing much if you skip it.