I've been visiting Gettysburg a couple times a year for the last 5 years, and have never had the opportunity to see the Cyclorama since its been undergoing a major restoration for the last several years. After finally seeing it in May '09, I have to say this is one of the most spectacular pieces of art I've ever seen. It's not just a painting, it's a 20'-ish tall by 400' wide painting of the battle of Gettysburg, and at the base of the painting the scene continues on a horizontal plane in 3-D. Very cool!
The only change I would like to see is that it would be nice to have more time to view the piece, and perhaps a more comfortable/relaxing setting. They corral the group into the viewing area, you listen to a narrative, then you are given a few minutes to look at the painting and everyone is ushered out. They also have some information on the restoration process which is also quite fascinating.