I asked my 4-year-old. She said it was too long, the story was good, and she really liked Mater the best (if you don't already know, that's Larry the Cable Guy). She did think it was funny, but some of the jokes, while funny, also were a little above her head. Example: "I'm happier than a tornado in a trailer park."
Also, I don't know if the deal is still on, but last week you could get a free movie pass from Toys R Us if you spent $20 in Cars toys.