Our engineering college has a career fair every spring and fall, I always went to them as an undergrad. The companies are mostly looking for co-op students and I did get my first co-op job because of going to the career fair.
Some look for full-time employees though, and before I graduated in 2004, I went to the career fair to see if there were any companies that were of interest to me. There was one company that had a huge line of CS students waiting to talk to them, but the guy for all other majors looked pretty bored. I stopped and talked to him and dropped off my resume. I got a call back about two weeks later to schedule an interview and ended up getting a job offer there. The money was good, the benefits were decent, but it wasn't really work I wanted to do, so I turned it down and applied for grad school.
I've got to think it's better than just mailing resumes to companies looking to hire you. The company wouldn't be there if they weren't looking for employees, and if you make an impression on the person there, they are likely to remember you when it comes to picking people to call back for interviews.