Massive Online Open Courses (MOOCs) are about two years old, at least the big three (Coursera, edX and Udacity) are. I've completed ten of them, I believe, and audited all the lectures of a few more. I may have started but dropped out of almost that many more. As long as MOOCs are free, the sensible approach is to try out anything that looks promising and drop out if they don't meet your expectation, or if you get too busy at work or home, or just get bored. Traditional educators are horrified by MOOC dropout rates, but the fact remains that a reasonably successful MOOC is completed by more students than many professors face in a lifetime.
At least two giant question marks hang over the MOOC at the moment: how to pay for them, and how do the students get credit for their learning. The answer to the first is becoming clearer: the students will pay to play. Udacity has already gone to a model where the student pays $150 a month to take a course. In return students will get some access to instructors and some credentials that may be worth something to prospective employers - or not. Coursera and edX are also trying to get students to pay for a chance to earn a "verified certificate".
The value of these certifications to a student is the second and even bigger question mark. Clearly the two questions are strongly linked. If the certificates are valuable, the students will pay. If not, they won't. At present, at least in the US, the standard unit of value is the degree, and right now none of the certificates counts toward a degree, so far as I know. That is a hurdle with a giant potential barrier. The institutions providing the courses don't want to devalue their degrees by offering credit at cut rate prices. Universities not offering MOOC courses have even bigger incentives not to grant credit that would undercut their own offerings by honoring MOOC certificates.
A third question intertwines with the first two: how does MOOC learning compare with that in a traditional brick and mortar course? Now that many traditional Universities offer online courses and degrees, how do they stack up against MOOCs. I've talked to students and faculty in the latter, and the online courses are pretty unpopular with both.
My own experience is that MOOCs can be very very good, but it requires a talented teacher and good production values - especially heavy investment in good online problems and problem software. They can also be strikingly mediocre - I dropped one course in an interesting subject when I realized that the lectures consisted of the professor reading his textbook in a nearly incomprehensible accent. It's also clear that not every student is suited to the MOOC. The sort of confident and talented student who gets into top universities can probably handle a MOOC well. So can amateur students with a degree or three under their belts and a certain amount of free time.
Still, nearly all the MOOCs I have taken are less demanding than their counterparts would be in a top school. Also, there is nothing like a MOOC curriculum right now. Instead, there is a scattering of courses, mostly at low levels, in things somebody would like to teach. Something like half a dozen courses on planets and exoplanets exist, but you won't find anything beyond the very elementary level in stellar astrophysics or most other astronomical topics.
I predict that MOOCs will need to be able to provide both structured curricula and real credit - probably including degrees - if they are to be a big success.