I'm also a grad of the Extension School, although not in CS. In-class instruction is fantastic for certain programs, with access to Harvard faculty, instructors, and other resources. I've blogged about it extensively here: http://harvardextended.blogspot.com/

Some undergraduate and graduate programs enable Extension students to attend Harvard College or GSAS classes. I don't remember the requirements/restrictions, but look up "special student" status on the HES website for details.