It depends on the fair. If you go to, for example, a job fair at a convention for your industry—that’s usually very good way to job search. If you go to an open job fair for a community, you’re less likely to find an actual job there in part because of the massive number of people who can (and do!) go. Also because I would send generalists to work the Chicago public job fair, but I send someone who knows that part of the business to the MBA women’s conference or the women in programming fair. Plus, at a very general fair, most of the jobs aren’t going to be stuff you are interested in—not your area.
My sister got her job through a job fair that was specific to her industry (she’s a software development engineer or something like that). She also got some call backs from that fair. Everyone was there for, basically, the same pretty specific thing so it was easier to figure out if a company was a good fit and get specific information. It was also easier to make contact with the people directly overlooking your resume.
With any job fair, you should be able to see what companies will be there ahead of time. Go to their website and look up what jobs they have to see if any interest you. You can even email them and say “I’m interested in blah and I’m going to this job fair, will someone knowledgeable about the area be there?” But usually less straightforward than that.