Did this on our first afternoon in Savannah to help see the city upon arrival and get the lay of the land (turns out that the way they have to navigate the one-way streets and all of the squares it didn't help imprint my mental map of the city).
The tour guide was friendly and enthusiastic, and certainly knowledgeable. There were two young costumed performers who hopped on at different stops. I know that was part of the experience but I'm not sure it added a great deal other than give our guide a break from narration at those stops.
Definitely a good way to see what's what so you can figure out what is of interest and merits more of your time.