^^^^^Please, don't get the wrong idea. The vast, VAST majority of all the guests I've met at conventions have been really nice people. That includes the attendees, as well. Most are sincere and appreciative. I've been able to engage in some really nice, stimulating conversations and built some great memories.
Don't let the negative stories turn you off from going to conventions.
I fully agree--you do tend to run into the odd guest who may be having a crappy day and therefore isn't quite as sociable as they probably should be, but by and large the actor guests are great.
I've also made a lot of great friendships from meeting people at cons--I have a whole crew of 'geek' friends who I've met solely through attending conventions, which is nice because a lot of my other friends don't really get my love of science fiction.
One thing to be aware of is the type of con you'll be attending--some are small, some are massive and some are well-run while others can be plagued with lack of organization and long lines. I tend to like fan-run not-for-profit cons better than the huge corporate for-profit ones, the latter can sometimes be far more interested in getting your money in their pockets than if you're actually enjoying yourself.
In the end I still love attending them, I'm going to three this year in Toronto, and if time and money permits I may make DragonCon in Atlanta the fourth one.