I started going to the career fairs when I had only 550 PIC or so. To an extent, it was good. I think that the advantage is that you can make some contacts, which never hurt. It also shows the airline that you are gunning for that you're interested. If they continue to see you at job fairs, then they'll eventually remember you when you do have the time, that's what happend to me. Another advantage is that the more job fairs you go to, you get better at talking to recruiters and know what questions to ask, and what not to ask.
I would say that the disadvantages are that they aren't cheap to attend! Plus, you have to consider travel costs. If you have jumpseat priveleges, great. However, there is still a hotel to think about, depending on the time of the career fair and if there are flights to get out on that night.
I attended 5 career fairs within 12 months. I must say that I'm kinda sick of attending them, but if it gets me a job, I'll be glad that I did it. I am planning on taking a break for a few months.
Good luck in whatever you decide.