I have been using K12 with my son for almost a whole year now. We live in NY, so it is not free to us (YET?), so the cost is something I want to consider for next year. For more than half the states in the US, there is a free public school option, though.
We chose to do the independent courses of Language Arts, Math, History, and Science this year. You can have guided courses with certified teachers as well, although there is an additional cost to it.
I like the process behind much of what they do (I was a certified school teacher in NY). We often skip things in the curriculum that I don’t feel my son needs, or they also offer extra activities if you think your student needs more to grasp the concept, or if the student is just plain enjoying it and wants to do more.
My son loves the History and Science curriculum, and I often use them as leverage to get him to do his other work. As a history teacher, I love the ideas behind what they teach, and it seems like there will be some repetition and building on concepts he has already learned as the years go.
There are some big ideas introduced already, like democracy, differences in religions, and specific scientific tools that we have built and used.
The website offers outlines for each course, so you can look to see what main topics will be covered. I have felt like the curriculum has pushed my son to work diligently, but not beyond his ability. I have not used any other prepared curriculum before, so I do not have a reference point for the amount of work required of us.
For Kindergarten, I created my own curriculum, so I was very happy to have things laid out so well for me this year.
They help me to keep track of my schedule online as well, so it helps with my quarterly reports that I send to the school district.