I can't claim to know Mark, but I have met him once, and had a brief conversation with him. I also got to listen to his keynote at the event and see him interact with the rest of the people who mobbed him trying to get a word (or a pitch) in edgewise after his keynote.

My impression was very favorable, to be honest. I thought he seemed like a laid back, down to earth guy.. not at all like what I thought a billionaire would be like. He was just a guy in jeans and a polo shirt, hanging around in front of the stage talking with people. If you had not known who he was, there was no clue that he was ultra rich or a celebrity, other than the crowd milling around him.

FWIW, his response to our company idea was basically "sounds good, you just have to execute". :-)