Well, If you see the 2D and 3D version of the movie isn't that 5D? Hey, they all can't be winners!
At any rate, I had the luxury to see both versions of Monsters vs Aliens (for free of course) just a few days apart.
I saw the 2D first then upon enjoying it and realizing that some parts would look really cool in 3D, I went to see it again. Let me say that the 3D felt like an "enhancement" (or maybe whipped cream on top of an already tasty dessert) to the already good movie.
As with all of my movie reviews, Spoilers may be mentioned.
Keeping in mind that the target audience are kids, I think the guys at Dreamworks put enough jokes and classic movie references (such as The Blob, or B.O.B.'s origin or the use of the Close Encounters theme) to keep adults entertained. It's a simple story about Susan (voiced by Reese Witherspoon) who is hit by a meteor and becomes the 50 foot woman. During this time, there is an alien invasion and Susan (now called Ginormica) along with her fellow monsters must defend the earth. Like I said, simple.
The movie was funny, the animation was top notch (even in 2D, you can appreciate the depth and the movements of the characters) and above all, it was entertaining.
If I had any complaints it was the under use of Insectasarus (a Godzilla-type monster, eats Tokyo). For his size, his abilities were lacking and even tho he did save the day (or at least our heroes) at the end, I felt he could have done more against the giant robot in the initial battle. Then again, it's not his story, it's Susan's.
So go see it. If you can't afford the extra $$ for the 3D, it's still a good 2D movie.