I took two online classes, Spanish and Comp, my junior year in high school. Comp I had to drop out of because it was too much work coupled with Spanish and my regular schoolwork (I was home schooled).
Compared to taking Spanish II (senior year) in an actual class, I much preferred real class to online. Pretty much what Broby said, though I used online class as an excuse to be on the internet way too much (most of it was spent on OldGuild). I wouldn't know about taking all classes online, I probably wouldn't like it because I'm more of a hands-on learner and I would get bored. Just depends on your personality I guess.