I've also only been to one once. Leonard Nimoy was there. It was cool.
Only, I was around the age of five when that happened. Reflecting on the event as an adult, I think it might have been a guy who put on Spock ears, then talked about Leonard Nimoy. I'd never even seen Star Trek at that point; I wouldn't have known the difference.
I actually have a second memory of visiting one, but I think that was just an episode of Friends. I guess I was pretty gullible as a kid. (And totally not now.)