We used Calvert Pre-K when DD was 3. It was okay. I have heard from several reviewers that it's boring in K and up, though, and you can't get different levels for subjects, except for math. All the language arts, history, science, etc are lumped together for their grade, so if you have child gifted in science, he wouldn't be able to work ahead unless you supplement with something else. I have heard that Calvert students become pretty solid writers, but these reviews were from parents of ND kids, so YMMV.
