At times I wonder if I am missing out on the graduate student experience by being a part-time, online student. I am on the graduate student distribution list, so every now and then there are events that are specifically for grad students, and some of these events I would really enjoy. I think I miss out on the grad student social networking. However, even if I was on campus, I don't think I would be living the full grad student life, unless I went full-time. I think full time grad studies are like undergrad, but with better hours.
I don't know the statistics, but it would seem that most people are part time graduate students because they need to have a job in order to pay for it and the previous student loans that so many people seem to have. Also, some programs encourage their applicants to have some work experience before they attend graduate school. I am sure that there are benefits of going straight from undergrad to grad school without a break. One definite plus is that your rhythm is not broken, meaning you are still used to going to school and doing homework and projects. Many people after they start working full time, find it too hard to get back into school mode. When this happens they keep putting it off and delaying it further and then "life happens" and they never go back.
There are a lot people like that at the company that I work for, many people just never found the chance to go back, or are 3 classes or so from finishing up a degree, but never get around to it. Either way you choose to go to grad school, full time, part time, online or on campus, finding the right fit for your circumstances will ultimately determine whether or not you complete your degree.