I went to the New York one too. Most people were dressed business casual. There were a few people all decked out in suits, and then a few in ripped jeans and Uggs. It probably doesn't matter, but I'd be embarrassed if I didn't look put together.
Come prepared with questions for each of the schools you're interested in. I only had questions for a couple of schools, and at the rest I was just introducing myself and just grabbing viewbooks. This is fine too I guess. They expect a lot of people just to be gathering info, not deep into their research yet. I only got one fee waiver, but it was from a school I was planning on applying to anyway, so it was cool.
I feel like I got a better sense of some of the schools, and slightly changed my list of schools to apply to. I only spent an hour there, out of an 8 hour day because of transportation. So it could have been a waste of time and money, but I think I got enough out of it.