Amy Allen (Aayla Secura from Star Wars Ep II) If you can even call her a celebrity, has been unpleasant every time I have ran into her at cons. I can't give any specifics, I just find her extremely unpleasant in person. There are a few other stars I can think of like that, but she is the worst I have met yet.
On the other side of things, Peter Mayhew, and Warwick Davis, are two of the greatest guys I have ever met. I don't just mean celebrities either, I mean, they are just great guys. I can't say enough great things about those two. I am always happy when I here they are going to be at a con.

I have stood in line for both their autographs many times, and I have seen them both speak at cons a number of times. I have also road an elevator with Peter.
At CIII (I think it was CIII) Wall I was standing in line to get autographs, my Dad sat on a bench to wait for me. I was in line for a long time, and I forget who I ended up getting an autograph from. When I was done, I walked over, and found Warwick Davis, and his wife, setting with my Dad. No Sucrets, No handlers, or anything. My Dad just said, "Well see 'u leter." to Warwick, and walked over to meat up with me. After my Dad got to me, I said, "Do you know who that was? " And Dad said "who the little guy?....... NO, is he one of these big shots." I told Dad who it was, and Dad just said "Hm, he, and his wife just came over a sat with me, wall I was waiting. We had a nice talk. He seamed cool."
Jeremy Bulloch is pretty cool too.