I got the book when DS was about 11. He was way past preschool years and I could not remember most of his early development so the checklist had not been very useful to us. What I found really useful in the book was the anecdotal descriptions of each of the kids at each of the levels, especially at the older ages. I found that as I read each chapter through with very concrete descriptions of what the kids were doing, I got a much better overall sense of what they were really like. When I got to the section on a certain level, everything snapped into place and I though, "Yep, that is just like DS." It was a really great moment for me.
There are lots of methodologic issues as kimck says so I don't think it is the be-all and end-all, but for us having the actual stories of actual kids in front of us presented somewhat systematically was a great reality check for us.
