This is really a rather well-done movie.
Humor factor: Lots of jokes for the adults that the little kids will miss. Lots of slapstick and goofiness that the kids eat up. Several references to other movies, including westerns and Star Wars - with Ewan MacGreggor doing the voice of the lead, this is not surprising.
Evil factor: Greg Kinnear plays his part very well, with a snippy attitude and a hint of neurosis. Self-image, economics, and eugenics/genocide has never been candy coated better.
Goof factor: Robin Williams, Mel Brooks, Amanda Bynes, Drew Carry - they all really do a kick-ass job with their characters. Many reviews I've read have ranted and raved about Williams - in his first animated role since Aladdin.... but really I felt his part was a bit subdued - which was a good thing. Had he been over-the-top in this movie, it would not have worked (whereas it was what made Aladdin).
Being that I'm more just rambling here, I'll tell you that the boys loved it, and I was not at all disappointed with this movie. Well worth the money and popcorn.
I give this 4.5 something out of whatever, because well, yeah. I liked it.