My dad was skipped 3 grades throughout his school years. He ended up going to college at 15. All of his accelerations were school initiated. He definitely thinks it was a good idea because he remembers what it felt like to be bored in class, even after the accelerations. He describes extreme frustration with teachers, other students, and himself. Also, he was in trouble A LOT!
He did ok until he went to college, and that's where the positive experience of his acceleration stops. He was not mature enough to handle the independence of college at such a young age.
Of course, I think in this day and age, we have lots of other opportunities for kids who finish high school early: distance learning, online college courses, local community colleges or universities near home. My dad just went away to college like most of us do at 18 or 19 and only came home on holidays.
All in all, he ended up doing fine, but he says he made decisions he regrets because he wasn't equipped to handle the responsibility. That said, I'm still in favor of acceleration, whether it be full-grade or subject.
I know my son is much happier with older children anyway. He has a hard time relating to kids his age, and there are frequent conflicts. With older friends, this problem is non-existent. In his case, acceleration would be great. If only I could get him to take a risk and really show them what he can do!