I think he says some interesting stuff.But I don't really like him. Last year he went on a a SME that was suppose to hit Earth this year, and that congress should be spending billions preparing transformers for it. And it still doesn't happen.
Yes, he's brilliant. He also seems to be prepared to talk about literally anything as long as it gets him on TV. Once they start talking about how to actually build a time machine, you're on rather thin credibility. (FTL drives are a lot more plausible, since we already know at least one way to do that, assuming you could fix the horrendous technical obstacles, which NASA seems to think they can.)
I always though he was an eccentric really: his ideas could be a bit unusual at times. brilliant a lot of times though