I swear they keep the lines long on purpose to spur membership sales.

It worked with us. I looked at the line, calculated the value of an hour or two of my time waiting in the hot sun to get in, and decided the membership was worth it.

And we have used it multiple times

The Shedd (as well as the other museums in Chicago) offer opportunities to spend the night in the museums. As a cub scout leader we arranged for our group to spend the night in the Shedd, Museum of S&I, and Field. We slept in the various spokes of the central hub. at the time we got a pizza and salad dinner, a snack before lights out and a "continental breakfast". Trust me you don't go for the food, but the experience is something that you and your child will not soon forget. Many different types of groups were there not just cub, boy and girl scouts.

We got "Sleeping with the Fishes" and "Dozin with the Dinos" T-shirts too.

Least favorite was the Museum of S&I, they awake you at dawn with the sound of the steam engine whistle and never really turn down the lights.

And I believe the Planetarium offers overnights as well.