It's legit, but unnecessary. Career counseling firms are a different story, but that's another column. Career Fairs Can't seem to go two weeks without one anymore. Almost the equivalent of singles bars for jobseekers. No, really, there's more to job fairs than aimless wandering or ten minutes of pointless conversation with a clueless staff recruiter.
Career fairs are another excellent way to test the water.
Good companies send tech people as well as employment reps. With any luck, you'll actually be able to learn something about the organization, what it is trying to do and whether it's a place you might want to work. If the company has sent someone who doesn't know anything about technology, you can ask for the name of the person who would be in charge of the group you'd be interested in. Be able to explain what you want to do in layperson's terms. As you've no doubt experienced, non-IT people can be remarkably unaware of the differences between IT occupations.