Have you met any celebrities in person that you found a disappointing or nice event? Not just in conventions, but also in regular circumstances, which can make a difference. I can understand some reasonable celebrities ignoring annoying pestering for autographs in the shopping queue or politely saying "Sorry I'm in a hurry" if somebody calls at them in the street, since celebrities are people too and you can't just suddenly walk up to them in their free time (which is why I don't sympathise with members of the paparazzi getting assaulted). But celebrities being very rude to bystanders for no reason at all (I heard of a late female sports presenter flying into a tantrum at somebody in a shop because he crossed her path) and being unpleasant in formal gatherings (like fansignings where the celebrities are obligated to be polite) is a sure sign of them being genuine assholes.