I am currently taking two Coursera classes (one is on probabilistic graphical models and the other on NLP) and so far at least, I am impressed by the depth of material covered and general quality. I will probably need to spend 10 to 15 hours a week on each class.
There are some subjects that I am interested in that I would like just a lighter weight overview, get a lay of the land - I think that this is what Udacity is for.