I have never used much websites for the on-campus courses, they are rarely self-contained and require more organization and research to use. There is one good feature of those: they often have excellent assignments that you often won't find in textbooks, so I think can be a good complement to a textbook. One thing that bothers me though is that these can go down at arbitrary time and you can't be sure it works properly. E.g. a website for 2013 course may have some of the links broken and you will only find out when you get there. Still, better than nothing and I appreciate greatly such initiatives.
>How has the social aspect + taking the course with others helped you in the past First of all there was just a general excitement "A course just opened and there are like 40 thousand of us". If you spend time around people that would do things like that than I guess the effect won't be as striking. But 40k was like a half of the population of the town I lived in. It felt like a part of something huge.
In general, thinking that many people go through the exercises right now and struggle just as I am was somehow motivating and uplifting. Reading that something had the same issues as you 5 years ago vs right now somehow make quite a bit of difference for me.
Then there is interaction. Getting your questions discussed and helping others on the forum is totally different for me than just browsing through a dead forum section for a possible answer to my problem. I had a technique: when I completed something I would go to forums and read questions, and then try to answer them at least in my head. Doing so with a "real" questions that are waiting for an answer is more engaging because in addition to practice you also help someone.
And the most insane thing that only happened once: competition. I was doing Sedgewick's algorithms and the automated grader itself was brutal. I loved the course so much that I took it twice, the second time doing all assignments to 100/100. But in addition to the grader there was forum section where people would show their stats (how fast their solution worked). I remember cutting my time only to discover next day that someone did better, and then half excited half suspecting that the person simply trolling with fake numbers would go and try to optimize my solution even further. I would wake up at 5am to work on it before work, the whole place is messy with drawings on paper, etc. I cannot imagine doing the same thing when reading a book.