Is the Sears Tower Skydeck worth a visit?

Yes.

But if you visit, take your time. Enjoy the build-up to the elevator ride by watching the movie, visiting the computer kiosks in the queue, and learning landmarks that you can identify once you're on the sky deck.

I think going at sunset is the best time to visit the skydeck. Watching the city lights take over your field of view as the sun goes down over Chicagoland on a clear winter night is a sight to see.