Well, we've done a triple skip.
It's been fine. More than fine, actually.
We went through some really rough spots during 8th-9th grades, though-- that is when the executive skills expectations seem to be most out of whack with HG/PG maturation at ages 8-12. As Val notes, it's VERY awkward for not-yet-adolescents to be surrounded by all that hormonally-fueled adolescence. It improves as the child ages into adolescence.
We were very uneasy about our DD's matriculation into a college environment at age 14 at that point (when she was 9, 10, 11). Now that she's only 18 months away, though, it is far less daunting.
She has friends, they don't seem to really care that she's 3-5 years younger, and she is content with her AP classes. Happy, in fact.
One caution that I'd offer when someone begins considering a 2+ year acceleration is that this may well change the picture considerably for college. For us, this didn't matter very much. Because of her disability (hidden, medical) we already planned for her to have the option (strongly encouraged, in fact) to stay at home for undergrad, even at age 17-18. So now she'll be 14 instead. Oh well.
Sending your child 3K miles away for undergrad is almost unbearable if they'll only be 14, 15 at the time, though. It limits you to one of just a handful of programs intended for kids like that, and maybe that is an okay thing-- but maybe it isn't, too.
