I have a friend who has been to many conventions and these are his takes:
Nichelle Nichols: he, his wife and his mom actually had dinner with her at one of the conventions (1990 maybe?) and she was an absolute joy. He said she was charming, funny and witty, very interested in the fans and their own stories, no problem with autographs or pictures.
Patrick Stewart: large picture taking session, friendly but said "I wish I had time to chat more, but we have to get through this large line. Thanks for coming." Very nice.
My own experiences: Jimmy Doohan, George Takei and Walter Koenig were very friendly and warm, no problems. Leonard Nimoy and William Shater waved to everyone and said thanks for coming, etc. Nothing negative to report.
My wife and I were at one of Terry Farrell's first conventions and she was either high as a kite or drunk with bloodshot eyes and constantly dropped f-bombs into her talk to the crowd. One of the convention vendors said no one could keep up with her partying! A husband and kids later, she seems to have calmed down a lot, judging from the above story of her going out of her way to make sure everyone received autographs.