I only started going to conventions recently. I saw that William Shatner was going to be at a local Trek Con in the Boston area two summers ago and thought, "He's 81. This might be my only chance." I'm glad I did. Got to meet him and got a picture. He was soft-spoken, but friendly.
I've been to a bunch of cons since, and met some friendly folks, and a couple not so great. People are people, and some have bad days.
By far the best has been Patrick Stewart - friendly and warm. He got my son giggling while we were taking a picture with him. All he did was introduce himself using that voice of his, "Hello, Jacob. I'm Patrick." and they started laughing. They were having the best time and all I could think to myself was, "Holy shit! Patrick Stewart's playing with my kid!" The camaera man finally had to break them up so we could get the shot! I shook his hand and thanked him. He was very gracious and wished us well.
I met John DeLancie - He was really nice. Asked what I did for a living and talked about music. Max Grodenchik was friendly - I helped him find the train station. Chase Masterson was a bit stand-offish. Robert Picardo was friendly. Met JG and Robert O'Reilly as the Klingons - hilarious!
Non-Trek people: Met a few of the original BSG cast. Richard Hatch is kind. Dirk Benedict is crazy. Herb Jefferson Jr. is a great guy - we spent a lot of time talking. Stan Lee is 90 and having the time of his life. He gets the scene (and gets paid big bucks!).
The only poor experience so far has been Bruce Boxleitner - All I will say is he wasn't nice to fans or the people working the con.
After reading some stories here, I am really hoping to meet Avery Brooks at some point. He seems like a great guy.