The Willis Tower (formally Sears Tower) is an essential part of Chicago's iconic skyline. If you want to be a apart of that skyline the Skydeck is the way to go.
For the best experience arrive at the building right when the Skydeck opens. Wait times can be hours during the afternoon on busy weekends. If you do get stuck in line make sure to partake in some of the fun interactive displays to pass the time. Ticket prices are high but definitely worth it.
Once you make it up the elevator to 103rd Floor the views are amazing. But, for most visitors (including myself) The Ledge is the best part of the trip. It consists of 4 all glass boxes that you can step out onto. They jot out from the building and let you look straight down to the sidewalk below. It is hard to take the first step out, but once you do it feels like you are floating.
TIP: Don't buy the expensive souvenirs from the shops inside the attraction. Head on out and there are many shops on Michigan Avenue and State Street.