I haven't, but can you be more specific and I can try and find something or maybe someone who has can give you feedback.
I will be using mostly kindergarten level subjects, and 1st or 2nd grade in reading. I plan on using math, reading, science, and social studies for sure. It is ordered off of the website.
We used K12 through a virtual school for some of last year. The only thing we liked was Social Studies (kindergarten level). It consisted of learning the names and positions of the continents (with a song), and introduced various countries. I really didn't like k12 but maybe that was because I was going through a virtual school and they were not very flexible. For instance, we joined 4 months into the school year and they were making us do ALL THE WORKSHEETS in all the subjects to catch up along with the work currently due. Four months worth of work! Crazy! It was all about paperwork with them. Get it done-push-push-push! Anyway, we quit with them. If you can do it at your own pace that will make a difference. You can tear out what your student already knows or otherwise alter the assignments.
Those are the comments I hear most of all. I know my sil was frustrated with the freedom I had to set my own schedule, and she wasn't.