they're fairly rushed and cramped. i went to the one in nyc last year, and it was really annoying. there were tons of major ass-kissers pushing their way to the front, dressed really nice, apparently under the impression that some admissions person is going to remember them over the other 3,000 (i'm making that number up) people that showed up that day. the only thing they're good for is picking up colorful brochures from all the schools you're interested in. i found the answers that the admissions people gave to be really bland and generic, and not helpful at all ('we look at every application thoroughly and take blah blah blah into account...').
on the flipside, as long as you register, you may receive app fee waivers from schools. william and mary offered the waiver last year to all nyc registerees.