That's about my experience too, having done only two classes so far. One of the largest downsides (in my experience) is never meeting/hearing from the instructors of the courses- the TAs are almost solely responsible for running the classes. I went to a large state school for undergrad, so lack of instructor facetime has not really been an issue for me. I've also found many of the udacity lectures to be so-so, but the GA tech projects really help to drill information into your brain and can be quite time-consuming.