I'm doing an online CS masters' program. I was in a traditional math PhD program for a few years. I miss hanging with fellow grad students, just because there aren't many opportunities to talk to people who care about the kinds of things you go to graduate school because you care about outside of a university, but I interact with my professors about as much as I did when I was going to physical classes. I don't think it would work as well as it does for anything but computer science, because a good chunk of what you learn in computing you learn from the machine itself -- you don't need nearly as much human feedback for a program as you do for a proof or a paper, but it works pretty well for computer science. About the only complaint I have is that online proctoring services are horrible.