I've been to two admitted students days (and will be going to another in two weeks), and I found them to be very helpful in sorting out my subjective sense of the schools. The first school left me with kind of ambivelent feelings, and the second one really fired me up after going there. #1 is now off my list. There was nothing specifically bad about #1, but it just didn't do anything for me.
I think it is a good way of figuring out where to go, because I imagine that being really excited about the school when you start will be a nice springboard to doing well in the early going.