I've bothered to follow up on some of these businesses, and I've yet to miss anything substantial.

But in the absence of any other data, NDA becomes a very good indication of whether I should continue the conversation.

Can someone run off with an idea and make a billion dollars? Maybe. But I would suggest that an NDA gives very little protection. If the person is going to do that and actually manages to make a billion dollars from it, then all you have is a piece of paper saying the person won't discuss the idea without specifying the idea.

Unless you then have a paper trail of discussing the idea, how can you argue that that was your idea? Most subsequent discussions in my experience have been verbal.

Worse still they have a billion dollars to fight the matter in court.

I still think about him sometimes and wonder who he's bothering now.

Now, if you have a huge name and track record... I can get it. Years ago talked with someone who'd done some deals with the 'guerilla marketing' guy (Levinson IIRC). After a while, he was more or less just licensing his name/brand for a cut, but it was worth it because there was already a track record and built-in audience - anything with his name/brand would be guaranteed some degree of sales on day one.

Most people who've approached me with ideas like that haven't had that success. Even in one case where I was working with a guy who had executed multiple businesses and had good success (one 8 figure exit) - we just couldn't work together - our expectations of each other never synced up.