Online courses, particularly massive open online courses (MOOC) are the current rage. I can see that having students going through the material presented at their own pace, whenever they have time, and to be able to "rewind" are valuable. But I think some form of one-on-one contact (even indirectly, through grading, commenting on the work, answering questions, etc) is required anyway.
So, the question is: What are experiences in this area? How much (or little) extra work (contact with students, grading, ...) is required? What part of that (if any) can be left to teaching assistants (or similar)?
Perhaps deeper: How does this compare to more traditional technologies, i.e. a good textbook (perhaps even one like Bogart's "Combinatorics through Guided Discovery") and self study, seasoned with questions or just browsing http://math.stackexchange.com?