Perhaps i didn't enjoy this because i'm not a kid anymore.. i dunno, i may be an 'adult' but i still love kids films, so i'll just tell you why i didn't enjoy this film as much as i felt i should.
Basically, the idea is great, it could have been amazing, it had potential. However it was just poorly done, the humor was average, but not funny enough to laugh out loud, the characters weren't that well developed and by the end of the film you struggle to remember it, even writing this now i have no idea which bits to refer to.
I would recommend this film for young children, they won't notice a bad storyline as i did, as a film to rent and watch as a family, it's ok, it'll pass the time, but it will be tedious for adults.
As for me, i wouldn't watch it again, though i do hope that in the future they make a film with the same premise as Planet 51, because like i said, it had potential, it could have been a truly awesome film.