State Sponsored K-12 Program
I have been using the K12 curriculum through the CAVA program, which is the California Virtual Academy. Three of my children have used the K12 curriculum for the past 4 years and we have been pretty satisfied with it.
To begin, we contacted the California Virtual Academy, submitted all the pertinent information, and soon received all the workbooks, reading material, science experiment materials, etc. through the mail. Since most of the lessons are online and completed work is noted online, we were also sent a computer to connect to the internet.
Via the CAVA program, which is state funded, this material has been free to us year after year. The K12 curriculum is also available for sale to homeschooling families that choose not to use a state program, or those in states that don't offer this option. The costs of the workbooks and other materials would probably have been too expensive for us to consider otherwise.
Many states are now offering a similar virtual academy, however, and through those programs these K12 materials are also available at no cost. The K12 curriculum is quite complete in it's coverage of subjects, which are math, history, vocabulary, grammar, art, science, music, and language. And like it's name implies, the K12 curriculum spans from kindergarten to grade twelve.
The art and science sections have particularly nice lessons that I believe my children have truly enjoyed. The art literature has been fascinating, both to my children and to myself. The science experiments are occasionally pointless, but usually interesting enough to generate enthusiasm from the kids when science time comes around.
As to the faults of the K12 curriculum, from our perspective they are relatively few, and most likely due to the standard curriculum of California school. Notably, the music material has been tedious at times. Our oldest also laments that the history lessons seem to focus on Greece year after year.
There does seem to be quite a bit of material to cover some days, and once in a while we struggle to get all the assignments completed within the correct time frame. I haven't had experience with any other homeschooling program, but overall I have found the K12 homeschool curriculum to be very sufficient.