I visited one of the four schools I applied to - after I had submitted my application, but before I heard back from them. I had already been accepted into another school, and was getting ready to make a decision in case I got in to the one I was visiting (I received my rejection e-mail from this school a week after my visit).
The visit started with a Q&A session with two current students. The Q&A session was marginally useful, but didn't produce any earth-shattering insights. Next, there was a presentation by a woman from the admission committee who walked us through the school's curriculum, specialty tracks, and job placement statistics. This was followed by a second Q&A session that also didn't produce any earth-shattering results.
After the Q&A session was done she wished us luck and said goodbye ... and that was it. Although I had already written my essay, I didn't walk away with any ideas on how I could have done a better job of tailoring my essays to this school.
I later spoke with a friend who had just finished visiting several b-schools that he was applying to. His take on it? "It was a waste of time and money."
If you're looking for essay material, my advice is to not bother with a campus visit unless the school is near where you live or you're visiting that city anyway. Your time would be better spent at an information session closer to where you live or doing online research.
The one good reason I can see for visiting the school is to find out more about the city it is located in. If you're trying to choose between, say, New York and Boston, and you've never been to either city, it's probably worth the trip.