I visited Chicago in 2007 when it was called the Sears Tower. The views from the top were great; not quite what you would get from the top of the Empire State Building in New York, but close.
Now the tower has been renamed the Willis Tower and to attract tourists, they have brought a vertigo inducing experience to the 103rd floor. It is called Skydeck and is a glass box that extends over 4 feet out over the side of the building.
If you visit the Skydeck and still feel you need that adrenaline rush, check out the Grand Canyon Skywalk hanging 4,000 feet above the Colorado River.