I'll come back to this when I've got time to find the details, but in short it is much more common in the US, and so most of the research comes from there. The main fact I can remember, is that the single biggest factor in determining whether a grade skip will work or not is the commitment of the school to the move. Which is quite possibly why it's not often successful over here. But they also use a thing called the Iowa Acceleration Scale which uses a whole range of factors, including maturity, to see if a child is suitable for acceleration. It's worth bearing in mind that a very gifted/bright child may not have that much in common with his own age peers either.
And there are side effects from not accelerating as well; a good percentage of very gifted children are home educated, because there is no way that they can be taught at the right pace or level within their own year group.
In this context, I think AMumInScotland's comment about music groups is quite telling; there are plenty of subjects, not just music, where children would benefit from being able to work at their own pace.
But it's not necessarily the best outcome. As other people have said, a really gifted child will soon outgrow the skip as they are learning faster. The best set up for gifted children is when they are taught with a similar cohort. Or, in other words, a grammar school.
And the reason I've looked at this is that DD is one year grade skipped in primary, and she, the school and the ed psych are all agreed that it is the right thing. And that's not just academically but social. I was so pleased last half term. For the first time in her life, she played a shared imaginative game with her friends, based around a book they'd all read. That just didn't happen before.