I graduated from the program in December and I found it incredibly rewarding. There are a lot of great classes, and I learned a ton - in particular, the machine learning and reinforcement learning courses were top notch, as we're the systems programming ones.
One thing I'd warn though is that you'll get out of the program what you put into it - so it's really up to you to choose classes that will set up your career the way that you want it.
I'm about to start my 5th semester (one more to go after this!) in the OMSCS program while working full time. From my perspective, this totally depends on several factors:
1. Your background. This will seriously affect your time spent in many classes. For example, if you don't know C and take a class that uses it, you're going to spend way more time than someone already comfortable.
2. Level of effort. Most projects have a rubric where you know what to complete for what kind of grade. If you do the least to pass, that's obviously going to take a lot less work than going for the A + extra credit.
3. Interest level. I find myself spending more time (sometimes more than I should) on the classes I really get into than ones I have to take to complete some MSCS specialization requirement.
4. Instructional team. As in traditional programs, some course teams are better organized than others which can impact how much time you have to spend staying on top of material or actually getting ahead.
In the end, it would be good to check out the OMSCS Course Surveys (https://gt-course-surveys.herokuapp.com/#/) to see what people report for weekly hours spent vs the factors above.
With one course, it was challenging but I could fit it into my schedule without too many changes. My schedule was to watch lectures during the week in the mornings before work (45 minutes or so a day), and then do homework in the mornings on Saturday and Sunday. Usually I still had enough time to hang out with family and friends.
With two courses, I had to be really strict about my schedule - basically, I'd go to work, come home, study, sleep, repeat, and usually use an entire day of the weekend to get things done. I got really good at saying no to people.
Yeah, most of the interaction takes place on forum software called Piazza. Some classes also do weekly Google Hangouts as office hours.
There are assignments, projects and exams - they've put a lot of effort into making it have the same quality and standards as the on campus programs, even going so far as to do blind grading in some courses (i.e. grade the on-campus and OMSCS courses as one).
One thing I'd warn though is that you'll get out of the program what you put into it - so it's really up to you to choose classes that will set up your career the way that you want it.