I hear they LOVE this. Shows your commitment - you dragged you @ss to school X, and when it wasn't even interview time! Well done.
I have put this sort of stuff in the part of the essay that asks how school X is going to help you achieve your goals. You know, "I learn best in an intimate cooperative environment. When I visited, I was SO impressed by how nice, non-nasty and generally okey-dokey your students are."
For more ideas in this general area, check out this thread, where Lhotseface had some dandies. (Good dandies, I mean. Not the, "whoa, that's a doozie" kind.)
http://www.gmatclub.com/phpbb/viewtopic ... 9&start=20