I've met several and spent a weekend seeing them off and on when I covered a run of science fiction conventions.
The nicest was probably Erin Gray. She was absolutely delightful to be around and totally a class act.
The most fun was Armin Shimerman, he spent most of the weekend telling stories and playing off the people. I think he has as much fun as the fans.
The biggest asshole, bar none, was Denise Crosby. She hated being there, didn't want anything to do with the fans who bought tickets and lined up to see her. And her video production company was out to poke fun at "trekkies" to boot.
The one who was probably the most like what you see on TV was Christopher Judge. Even though I had never seen "Stargate" when I met him, when I did finally see the show, I knew I'd met him being himself and not pretending to enjoy himself, just like he presented his character on the show.
There were others over the years, but those stand out in my memory.