The balconies are suspended 1,353 feet in the air and jut out four feet from the building's Skydeck.
The reward is unobstructed views of Chicago from the building's west side and a heart-stopping vista of the street and Chicago River below - for those brave enough to look straight down.
'It's like walking on ice,' visitor Margaret Kemp, from Bishop, California said. 'The first step you take you think "Am I going down?"'
'Look at all those tiny things that are usually huge.'
John Huston, one of the owners of the Sears Tower , even admitted to getting 'a little queasy' the first time he ventured out on to the balcony. However, after 30 or 40 trips, he seems to have got used to it.
'The Ledge is the world's most awesome view, the world's most precipitous view, the view with the most wow in the world.'
The balconies are 10ft high and 10ft wide, can hold five tons, and have glass which is 1.5 inch thick.
Architect Ross Wimer said: 'We did studies that showed a four-foot-deep (1.2 metres) enclosure makes you feel like you're floating since there's only room for one row of people, not two.'
The Skydeck attracts 25,000 visitors on clear days. They each pay $15 to take an elevator ride up to the 103rd floor of the 110-story office building that opened in 1973.