After getting through the K and 5th grade books in 5-6 weeks, I bought the whole set. My kids loved these so much they wanted to do more than planned for the day. The material is thought provoking and my kids learned a lot from the books. I recommend this series to all my friends!
My daughter has been using this series in 3rd and 4th grade. She enjoys them. I like how the material is simply presented in a story-telling fashion. I find the text very engaging. I think the drawings are pretty and add to the material. I use this as a journalling exercise. She reads a section and then writes an entry in her science journal about what she read and draws a picture. And she never complains about having to do it, so it must be good! We have the whole series and there is a lot of material there, so we can use it for several years.