I get tired of pointing these things out to people, but this is a comedy. People complaining about that fact that's it's unrealistic seem to be missing something. At no point did the movie makers expect you to find this realistic.
It's an escapist teen fantasy. Ordinary guy gets to drive expensive car, meet cute women, and have an adventure.
Of COURSE it's ludricous. But it's cute and funny in a lot of spots. I laughed out loud many times BECAUSE it's silly. So if you prefer sophisticated humor, maybe this is not for you.
It's less silly than say Top Secret. It's more of a straight action/adventure - it's just that the premise is totally unrealistic and so is the protagonist - he goes from being clueless to saving the day. Don't expect James Bond here people - it's not supposed to be.
I give it 7 out of 10. Why? Because it made me laugh out loud in multiple spots. Which is the point to comedies.
(Why some people watch comedies when they lack of a sense of humor is beyond me - some of the comments made here indicate that they do, though :)