The nicest "celebrities" I've met/know have been people from the cast of Xena, especially Adrienne Wilkinson who played Eve. Amber Benson from Buffy was really lovely.
I don't think I've met a not nice celebrity, although I was a little disappointed when I met Sharon Osbourne last year because I had been sooooo looking forward to that.
John McCririck (spelling!?) was surprisingly super nice, come to think of it.