I have been to about 3 job fairs. Most of them consist of Universities looking for people for PhD or Masters programs, Army & Police recruiters & most of the companies at them look mostly for Science & IT graduates. The last one I went to lead to a job interview that I had 2 rounds of interviews for just before Christmas last year but didn't get it in the end.

I find the key to them if you're actually seriously there to find work is to come across as charismatic as possible because chances are the people there recruiting may be your interviewer later.