I think Lola's post covered a lot of the same ground as what my husband has said were drawbacks for him. I'll see if I can lay them out (this is from memory, may be wrong):
1.) Lack of cohort. This may have already changed a great deal from when we were kids (70's), but for him it was a big thing that his classmates often had been together for x number of years and he suddenly popped in. I was with much of the same group from kindergarten through 12th grade, (and many from my high school went to my University, too -- there are about a dozen people with whom I have spent 17 years of my education) and that was really important to me.
2.) Physical delay. My husband is now 6'2" and strapping, but he was a normal-to-small kid, and at 1 to 2 years behind most of his classmates, really felt the effects of being a "shrimp."
3.) Emotional/ social delay. This is to echo Lola's last line, which I think is really important. It's hard to tell how emotionally/ socially mature your child is; things that we as parents see as mature (ability to carry on an intelligent conversation) don't necessarily translate.
4.) School quality. I think this one is really key, and is what really makes it impossible to give one blanket recommendation. My husband skipped a grade (or two... I really should know this but have forgotten
) but the school was such that even with the skipped grade(s) he was STILL bored.
The skipping didn't cure the boredom. Meanwhile, as I mentioned, my school allowed kids to study whatever they were interested in and to be as advanced as they could handle, and lots of us ran with that.