There's a real difference if you're "going to be schooled" or if it was your decision. There are several upsides to online school. For example, you can speed up in subjects that you're naturally good at and take your time on the ones that need more work. Public schools have everyone working at the same speed, but as we know people aren't the same. This is one great thing about online school. The other is the freedom to plan your time. You can easily make a doctor's appointment in the AM or whatever... things that would be complicated with the preset schedule of attending regular school.
About the downsides... one of the most important things to realize is that you still need to do the same amount of work. You can't sleep until 10am every day and think you'll get everything done. No, the best way is to start on a set time every day Mon-Fri, just like other students do. The other thing you have to do is that you need a schedule for yourself. Make one in Excel - plan on how many hours will be spent on each subject and what you need to get done (eg. essay deadline coming up, etc). This really is the hardest part - you need to be self-disciplined. In school you have teachers telling you what, when and how to do. At home it's all up to you and it's all YOUR responsibility. You need to take it seriously. You might even need to try harder than in public school cause you don't have a teacher in front of the class. You need to be the teacher as well as the student. I can tell you from experience that this is difficult and at first you might take it too easy because of your new found "freedom" and "time". It's okay for a few weeks, you need to adapt which is understandable, but do it too long and you'll fail. So remember - plan, schedule, discipline. But don't forget to de-stress! :)