My Take
I like that I can "build" books of content for my students to complete. This allows me to guide them more specifically and completely. I can also contribute to (or alter) existing content. This is a very powerful tool that works online, through iPads, Android, Chromebooks and Surfaces. This kind of flexibility can't be bought...because it's all free!
How I Use It
I really liked to be able to utilize the math lessons that were already created. Using the robust search function was of added benefit! This allowed for students to practice independently or as a group to confirm skills. Some lessons are a bit unclear as to whether a skill is mastered.