John Mayer didn't look super cool in that video, however, he does have a significant draw among the younger generation. So even if he did looks stupid in that video he would be an effective celebrity endorsement. If it was up to me, I'd say see if John Mayer is willing to public endorse Ron Paul.
Mel Gibson - No, Ron already has the potential racist problem from accepting the Neo-Nazi money, this will not help. Not to mention Mel is well pass his time and doesn't have a big draw.
Tom Cruise - He's nuts. Again no.
Leonardo DiCaprio - I'd say he would be a good celebrity endorsement.
Brad Pitt & Angelina Jolie - I personally don't like them... but I guess they would be good.
Personally I'm not so big on celebrity endorsements. I feel this way largely because a celebrity's influence, usually, far outweighs their knowledge. If I told you to vote for "person X," I would have to convince you that "person X" is a good choice by informing your of his/her positions, character, and ability to do what they say. If a celebrity says I support "person X" and you should to, you may vote for "person X" just because you liked Leo in "The Departed" (great movie btw) or a huge John Mayer fan. I find this damaging to a democratic society. Then again I find most other POTUS candidates far more damaging. So if a few celebrity endorsements play a role in Ron Paul getting elected, I guess it will be worth it. Somewhat machiavellian, but what are you going t do?