I had the chance to see his talk when I interned at Google, and then had the great privilege to have dinner and talk to him. He was an amazing man, very spry for a 79 year old. I'll never forget when he told us "If you've never had the experience of seeing a nuclear blast in person, it's transcendent." (I'm paraphrasing) Just hearing him talk about the the early days of nuclear research was just so interesting, I now wish I had recorded the conversation.
He was an amazing man.
Update: It would seem David had the same impressions