My Take
While there are limited history resources on CK-12, the sources they have are valuable. I like that there are multiple formats to read including PDF (which is great as a teacher if I want to project on the SMART board), or the kids can download the book to their iPhone or tablet.
How I Use It
Occasionally, students are assigned to "homebound instruction" or happen to be on an extended educational trip, which keeps them out of the classroom for extended periods of time. In a day when we had textbooks, we would just assign the corresponding reading to the students to keep them up-to-date with content. However, budget cuts have eliminated the purchasing of textbooks. I use the Flexbooks to assign readings. The students enjoy that they can either download the book as a PDF and print, or download it to their e-reader or tablet to read. I like that the content is searchable. The only issues I ever had with CK-12, was that the textbooks are mainly overviews and focus on general knowledge, rather than in-depth exploration of various topics.