We did the cemetery tour with Lisa Marie, and thoroughly enjoyed it. Lisa Marie was very knowledgeable about key sights within the cemetery, Savannah's history, and how the two tied together. She was great at answering all our questions.
The cemetery is huge, and two hours is not nearly sufficient to cover it all. But after the tour is done, you are free to roam around as long as you like. The landscape of the cemetery, as well as the graves, tombs, crypts, is all very picturesque so I would definitely suggest bringing a camera.
Be sure to wear comfortable shoes, and bring a bottle of water, as you will do a lot of walking.