It's a waste of money to go to those fairs. United, AA, and Delta all said at Women in Aviation last year "being here doesn't do anything to get hired, no one in recruiting even knows you're here. You get NO additional points for being here!" I was very disappointed to hear that, BUT, I did find the presentations informative (direction of company, future, hiring info, etc.)
They are great for the LCCs. They will interview from those who go to fairs.

Basically, they just go over your resume and tell how it looks. They also tell you a bit about what they are looking for and how competitive you may or may not be. United said, If you are a ROCKSTAR, you might get an extra point, but I think he meant NASA shuttle pilot, volunteer firefighter, war-hero, and you had to save his life sometime during the fair.....

But yes, if you go, dress in a suit and have each resume tailored to Delta and set your expectations LOW.