As funny as the movie itself!
It has some problems of course, like when the cars won't turn or the powerslide won't work, but the average of the game is really good.
A good point is that the landscapes are very rich on details, the whole Radiator Springs that you saw on the movie is there! Every car stares at you when you cross their way, they greet you, they complain when you hit their bumpers, the whole environment is alive! I mean that the visual of the game is just beautiful.
The missions in the game will garantee the fun because you won't be racing all the time, and the fact that you'll play with other characters apart from McQueen is another good point. The police pursuit, for example, is really cool!
I think this is a great game and everybody should play, single or multiplayer - since you CAN play the missions with a friend (really cool). Grab some snacks, turn you videogame on and have a great time!