I've taken dozens of courses on Coursera, have nearly completed my first edX course. What I've found works best for me is to learn relatively self-contained things, such as the Scala programming language, on Coursera / edX. For general CS/math courses, I don't find the classes as useful. There are too many issues with unspoken, inaccurate or unclear prereqs. Additionally, the courses force you to work at a steady pace, which often slows me down in some stretches and then eventually forces me to drop out if work gets busy. Additionally, some courses are significantly watered down.
Taking classes from MIT (or any school with a good selection) on Open Courseware fixes all of these problems. You do the exact same work as students did, you can see the same lectures, and you take the same exams. All of it is at your own pace. Best of all MIT knows very well which of its own classes are prereqs of its own classes, so the whole headache of jumping into courses you don't have the tools to take on is avoided, too.
MOOC platform => Library
MOOC course => Textbook
Another way of looking at it is as correspondence school. That's the University of Phoenix model. It suggests a track record of producing larger scale educational outcomes - degrees - rather than immediate vocational retraining via online distance learning platforms.
At the other end there's Codecademy. Basically, structured interactive tutorials. While offering vocational training, they are less likely to serve as a qualification.
Anyway, my experience is primarily on Coursera. It has been that I come out with a better understanding of computer science and programming, But any immediate vocational improvement is directly proportional to the time actually spent implementing the ideas in code - e.g. I took a course that reccommend EMACS and my 'skill' in EMACS is a result of many days using it.
Good luck.
I've spent a few hundred bucks on tech related courses and the value I've derived isn't direct. I enjoyed the courses and it keeps my tech skills current - I consider that a hobby expense as opposed to an investment expense.
You correctly identified time as a key parameter. I've signed up for dozens of coursera courses and not finished them. Those are free so no big deal. It would suck a bit more to pay cash for courses and not finish them.
Just because of this thing, I am currently employed. The people who didn't took such courses seriously are currently suffering or lacking of good job.
Without the pressure of grades going on your transcript, it can get exceedingly difficult to push yourself to finish all of the assignments, and there are few incentives against quitting after getting frustrated. This is why college courses work so well for so many people: good university transcripts are far more important than the certificates that online courses might offer.
Harvard's CS50x released some interesting stats after its first year. Of over 100,000 people who signed up, only 11% submitted the first and easiest problem set. Of these 100,000, only 0.9% completed the course. Of those who submitted the first problem set, 13% completed the course.
It's definitely possible to get a good education from online courses, but it's important to realize how difficult it will be to keep yourself on a schedule. Treat it as a normal university class, prepare yourself, and come up with a feasible daily/weekly schedule.
Are they graphics and productivity apps (e.g. Photoshop, Microsoft Office, etc)?
Or are they programming & databases development stack? (E.g. PHP, Ruby, Javascript, etc).
If the courses are of the first category (product apps), the videos can be very helpful. That type of training is heavily task based. In other words, your eyes follow the instructor's mouse as it moves around the screen and you "look over his shoulder" to learn how to use software. It's much easier to learn Photoshop from dynamic video lessons rather than a static book.
However for category of programming, it is more of a mixed bag. You will often get higher density of information and best practices from books and blogs rather than videos. That's not to say there aren't any exceptional programming videos but on balance, the books are higher quality. However, some programming videos are heavily "task based" such as how to navigate around MS Visual Studio, or Eclipse, or setting of Apache web server.
I wouldn't base your trade or part of your career on an online course alone. In order for the knowledge to stick you need to apply and practice it (like anything else including traditional classroom learning).
However in my experience MOOCs do give you a decent foundation, and likely put you in a position where you know of concepts being discussed in an area where your specific trade is being applied.
So my (perhaps unsolicited) advice is to not make the assumption that taking an online course will inundate you with useful knowledge to then go out in the industry and practice it. I think it'll give you a good foundation to either motivate you to gain practical experience or it'll make you aware that this particular trade is not a good fit for you.
Edit: To directly answer the original question, yes, I have seriously learned useful things from online courses.
That template: A course taught by the person who 'wrote the book', in this Odersky teaching Scala. Others on Coursera: Ullman/Automata, Wetherall/Networking, and Sedgewick/Algorithms. The great thing about these courses is that the instructors know what is important and what isn't and therefore the courses don't get bogged down stuffing every detail into the lectures or reflecting every detail in the homework assignments. The big ideas are the big ideas.
There are also courses taught by mid-career professors - e.g. not fully tenured. These tend to be pretty good, but there is an undercurrent that the course assignments must defend their employer's brand by being hard just to be hard. These also tend to be quality courses, though there seems to be more variation.
The last group are those taught by newer academics and these vary more widely in quality. They tend to take a less general approach as there can be the feeling that 'making a name for oneself' in a publish or perish world is part of the motivation. If the instructor's interest closely aligns with the course and the student's interests, then it can be really good. If not, then there's the 'unenroll' button.
More in depth: I definitely picked up some concepts in the lectures that I watched. I thought they were better than normal university lectures, mostly because you could skip ahead through them. But in general I think that for me, long assignments are a much more valuable teaching method, and most of the courses I followed/looked at had much too simple multiple choice fact-based questions for the practical part.
More webapps in the works...
But Udemy has great reputation. I strongly recommend you to take a course so as to be able to form your own opinion after all.