I was there about a year ago...unfortunately it wasn't open yet to go to the observation deck.
When I was in high school we went on a band trip to Chicago. At the Sears tower observation desk a couple of friends and I climbed up on the railing and leaned our heads against the window a la Ferris Bueller. My palms were mildly sweating until another friend came by and shook the railing. That was enough of that.