I'll just give you a little of my experience since we just did what you are considering.
I skipped my DS into 2nd this year. He was in K last year. His birthday was the day before the cut off so before the skip he was already the youngest in his class. That puts him probably 2 years younger than a lot of the kids 2nd grade. He turned 6 at the end of September. He has a step-sister in 2nd grade who is 8 and a step-brother who is 6 and in K so I was able to see his interactions with both age ranges before skipping him and I was pretty sure he'd do better with older kids socially.
I think for us it's been a really good decision. I'd say that he's still probably at least top 3 in his class (he's never brought home less than an A on a report card) but he is finally seeing some new material. Having to actually use his brain a little when doing homework has been a learning experience for both of us but I think that he needs to learn to put some effort forth in school so it's been a positive. We do have a problem with his handwriting being not so great but I think it's because his motor skills are still developing.
Socially he's doing really well. This is the first year (he's been in school since Pre-K) that he has friends that he talks about and he seems to have one or two that he plays with a lot. I think that the kids in 2nd grade are more on his level than the kids in 1st were. He also seems to have stepped up his maturity level now that he's in a class with older kids. He still gets in trouble from time to time but probably not any more than average.
I'm glad that we skipped him now so that he can move up with these kids through elementary and middle school. He's really short for his age so he's especially short for 2nd grade but nobody has bothered him about it. Nobody has said a thing about his age to him either as far as I know. This is the happiest he's been since he's been in school.
