I think that "consuming education" via video + interactive quizzes (and course-specific chat room & in person meetups) is much more attractive, and engaging, than sitting on your couch watching a DVD.

It is pretty incredible to know that learning a topic of interest is literally a few clicks away...

Also, on another note, it could be that since my cost as a student is almost zero (let's take the value of my time out of the equation) then I am more willing to "try" a class and, since the Udacity classes tend to be engaging, I actually stick around.

Edit: IMO Udacity is a great way to start learning something new, such as basic CS stuff.