I went to a small one. It's mad chaos, a ton of people.
It was really nervewracking, but it was easier than I expected to start talking to employers because they pretty much appraoch you. You just wander near their booth looking mildly intrigued, and they generally come over and go Hi, do you know about Company X?
xlb is right, everyone is in their own world. Be prepared to elbow some people. Basically I just put my resume in for everything I was mildly qualified for (not much). I got two job offers, so yes, it was successful. I wouldn't go to one again though, as most of the jobs there were things I was NOT interested in doing and SA causes me difficulty with, such as retail and cashiering and watching children.
Good luck!