Personal experience. He was screaming at some fans who went to say thank you for his work. I've heard other things from people who've met him and had similar experiences as well as people having worked with him. He's a bit bitchy about Star Trek now.
I'm curious too because I've definitely heard that he's a giant dick but never really looked into it. He did a great job with the Sisko and they even put some episodes in addressing racism in the modern world so too bad he's apparently a dick but oh well I guess.