We are currently using History 3, Science 3 and Art 3. (I also used the full first grade program, although we dropped music that year as well as this year--it's just not my thing! :)
I love how the art and history are coordinated. The science, IMO, is a little advanced for the grade, but we manage. I like choosing my own LA and Math, so I don't purchase these from K12.
I purchase K12 classes on my own, as I do not want to spend time jumping through testing/state standards/PS accountability hoops when I live in a where I don't have to report to anyone. It's worth the expense for me. Plus, I can teach two kiddos for the price of one, since they are using the same courses.
Next year I am planning to use Sonlight because I want to do American History, not modern, but I will still use a little K12. I may choose to skip K12 Art 4 and select their 5th grade American Art instead. I will most likely use their Science 4. I may also use their Science 1 for my rising first-grader.
Not sure if that helps, but we love K12. But I can see where it would be difficult (for us anyway) to get it all done for each child everyday. The full first grade load taught me that (and I wasn't even teaching anyone else at that time!)
Lisa