We started using K12 a month ago and we love it. But, we're independents, which means we can do as much or as little as we want. That said, I tend to have my DS do all of the exercises in the literature, history and science exercises b/c he needs help with writing and help overcoming his aversion to schoolwork. He meets the lesson objectives (they list them) from his first reading of the materials, but I view the exercises as good practice for him with school skills - something he definitely needs.
We like the curriculum b/c he is well-placed in four different grades, the subject matter is interesting, and it's very helpful for me to have a daily lesson plan with everything set out, as I work and I was having a hard time getting everything done.
The art course ties into the history and is great. He loves the science, history, art, and literature. I don't have him at the right level yet in grammar, vocabulary or math - too easy, and I just skim through the math looking for areas that weren't addressed with Aleks or Life of Fred so far. Not quite sure about the math yet, but I'll reserve judgment until we start the appropriate level (it's very hard to change curriculum as nothing lines up). Still, I am very, very pleased. But then again, I have the freedom of using the curriculum as an independent.
Re: the "busy work" thing and K12 - when we tried 3rd and 4th grade history, I thought there were hands-on projects that were busy work, but that some kids would find fun, like making a wreath and acting out Roman history, making mobiles, coloring, etc. We ended up taking History 5, which is much better for the work they ask for. Where 3rd grade history said write a sentence about what you learned in this lesson, and 4th grade history said write a paragraph about what you learned in this lesson in your history journal (DS would not like that), 5th grade history had better thought-provoking projects - like filling out sheets on document or photo anaylsis, write a newspaper article (not that he did that one), etc.
I also found the advanced life science to be more engaging than the basic life science. The basic asked to do online research and write a paragraph, but the advanced course is more hands-on and thought-provoking and clearly has the aim to create a science fair type project at the end.
Music is good, too - but the toughest for us. So far, I have to say that I like all the courses. The only drawback for us is that DS is really at vastly different levels within language arts, but their online computer tracking system can't break language arts into different levels.
And, fyi, most of the work is not on the computer. Advanced life science is 100% on the computer, language arts is 100% separate books and materials. Math is also primarily a text (Sadlier-Oxford 2000 - but being updated) and separate K12 written materials. Music sends a dvd, cd's and the lessons are on the computer but available in hard copies, too. Art is a mix.
If you're interested in K12 at all - find the appropriate yahoo groups. there is a K12 parent2parent group and a K12users group and a K12 gifted group. I'm sure you can get a response from parents in the same virtual school you're considering.
Good luck with your decision.
Here are the groups:http://groups.yahoo.com/group/K12Gifted/?yguid=393213987
(gifted - and some of you are probably on this list)http://groups.yahoo.com/group/K12Parent2Parent/?yguid=393213987
(this one is for independent users)http://groups.yahoo.com/group/K12Users/?yguid=393213987
(virtual school and independents)