One unique thing about The Story of the World series is that each book in the series has a higher reading level than the last, and so it can be used as a nice backbone for elementary through early middle school social studies learning. We've already completed the Ancient Times volume, covering prehistory through the Roman Empire. And then we took nearly a year to study the Roman Empire at its zenith. This year, we're delving into the second SOTW volume on the Middle Ages.
I've bought the test and the activity books that go with it. It's great to have projects to do and further reading ideas, but wow. If I do a tenth of what's listed in there... well, who are we kidding... no way I'm even getting to a tenth of it. It would take years. So if I wanted to, I could return to the same volume and have Emperor study it on a higher level, with additional reading projects and book reports and the like.
Another thing I enjoy very much about this series is that it doesn't look at world history as happening almost entirely in Europe until suddenly *zing* settlers appear in the New World. It includes chapters on the Aborigines of Australia and the Maori of New Zealand, the first people of North America, happenings in India and Africa, and so on.
I also have invested in Kingfisher's History Encyclopedia and I can easily see myself handing this book over to Emperor often and asking him to just look at it. The pictures in it are of very good quality and that is the ONE thing that SOTW lacks. All of its pictures are simple line maps and drawings.