I've completed a couple of courses at edX. Some were excellent, university grade. Like Introduction to functional programming (DelftX) and Artificial Intelligence (BerkeleyX). What these courses have in common is that you get to explore the material and figure out the details on your own while doing your homework. The lessons just cover the theory. Programming with C# (MicrosoftX) on the other hand was a little dull. Not bad if you want to get started with C#, but far less engaging.
If you want to further your career I suppose you have to go for certificates that are directly related to the move you're aiming for. In my case they just look nice on the resumé whereas previous work experience is what really matters.
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