I've been taking a Cryptography class at Coursera and it's been superb. I've also dabbled in a Coursera Algorithms class and it is also very good.
I haven't taken any of the classes at Udacity or edX, so don't have much to comment on there.
What's not clear to me is why there are suddenly multiple such initiatives. Why did MIT and Harvard feel they had to start their own initiative, rather than just joining one of the existing ones?
At any rate, it's hard to look such fine gift horses in the mouth, so I shan't. Thank you, modern institutions of higher education, for making these resources available to the world!