I've used various K12 courses over the years with my gifted son. To use them successfully with him, I need to be sure the level is high enough and then I need to get rid of the busywork. K12 is notorious for the inclusion of lots of worksheets that are fairly inane. However the core elements of their program--literature, science, and history (we've never used the math, art, or music)--are solid. I think there are better choices for gifted kids than K12 for the grammar, spelling, and composition strands of language arts, though.
There is a yahoo group dedicated to people using K12 with gifted students.
