I grew up a town over from John Mayer, who is from Fairfield, CT. I have met him several times before he became famous. I worked with the man who taught him to play guitar. And on all accounts, yes, he has a less than desirable personality. From people that have been around him since he became famous (I have no interest), he's the kind of person who is annoyed that you're fawning over him, and more annoyed if you are not.
BUT.
I do have a story about him that bears repeating and, if you give him a 0/100, perhaps you'll at least give him a few points for this one.
On Saturday March 12, 2011, John Mayer came to a local (for this area) place called Brennan's. I don't know why, because he must have known he would be instantly recognized going out in public just a few miles from his parents house, but he came out anyway. My friends were playing there that night, a guitar/organ/drums trio.
The people that were there were demanding that he get up and play which, not cool if you are the band playing there, but fine I guess?
ANYWAY, he takes the bait. However, he has a condition: pass a pitcher around the bar and fill it with money for the band. The pitcher was passed around, but, nope, Mayer decides there's not enough and please pass it around again, which they do.
Then he finally does play.
So, that's one nice thing he did one time that I and several people I know can confirm did happen.
Do what you will with that story.