To be fair, this O.E.R. is easily searched by subject and grade. There is a dashboard to help direct the students to stay on task. As a registered student user, you can bookmark where you left off, have your progress charted, create your own library, and chat with other users in your group. As a teacher, you can assign work, check progress, and have class discussions (using chat). Teachers can also use the textbooks that are available (grade 6 and up). The textbooks have videos for each lesson and an assignment. The pages are somewhat visually appealing and the quality is okay too.
CK-12 is mainly limited to math, science, and technology. There are some English lessons, but only for older students. There are not enough resources for the younger grades. A search for grade 3 animals, for example, had no results.
Would I use this resource? Not at as a primary teacher. For older grades it could be useful. The quality of the lessons would need to be improved. Having a game mode would be an added attraction. Students would like the ability to chat within their groups. Tracking progress would help students stay on track. The best part is that it is an open and free this site that is available to anyone anywhere. The textbooks can easily be updated. The site has all the markings to be a wonder resource in the future.
Are you as smart as a third grader? If you said 0.4 tenths as a decimal is 0.04 then you are. If not, maybe you should watch the video available on CK-12 for grade 2. :)