I think it really depends on the course and the motivation of the person taking the course. My experience with online courses primarily stems from online preparatory courses for standardized exam (I've taken my fair share of these, and still have a few to go).
Unlike small group, instructor-led courses, online course remind me of the old adage "you get out of it what you put into it."
I usually think of online courses as a study aid rather than a comprehensive learning experience - it's really important, in my opinion, to supplement them with learning from books, lectures, personal research, or whatever else.
I also think online courses are a terrible idea for high school students and younger. Students at that age usually do not have the focus or drive to use their computer solely as a study tool. You can thank the internet for that one.