For what little it's worth...
I had a great experience with being "sent up" two grades in reading, personally. And no one else went up. Just me! I was a mature little girl, but I suspect it also helped that I had grown up playing with a neighbor who was 2 years older than I was. (She wasn't in the same class, but the regular exposure to an older girl probably helped me.)
Aside from the initial nervousness about having to leave my kindergarten room and walk across the playground to the elementary school building, I think darn near everything about it was good for me! I wound up having friends of multiple ages as I went through school, and that seemed perfectly natural to me.
I never felt intimidated by the older kids. Maybe a boy/girl difference there? I always felt like the older girls took me under their wings. No one ever behaved badly to me that I can recall. (And as mouthy and bossy as I was [am!], I would have stood up to them and/or told on them if they had been mean to me!)
My mother recalls my crying once when my kindergarten class left for some reason while I was at reading, so I returned to an empty classroom. But I don't even remember that happening, which makes me think that it wasn't as traumatic for me as it seemed to be at the time. It certainly didn't make me sorry I did it, not even at the time. It felt very natural for me.
And as a side-benefit...When I was a high school freshman, I dated a very nice, very gentlemanly senior boy and felt perfectly at home with him and his friends. Given the difference in maturity levels between freshman girls and freshman boys, I was really glad to date an older guy. Looking back, I wonder if I would have felt so sure of myself around his friends if I hadn't had the experience of being "sent up" through elementary school.
I finished the entire series of reading textbooks the school used with two years of school left to go. Rather than repeating, they got a couple sample texts that they had never ordered before and I did an independent study with them during 5th grade. When I finished those, I just read whatever books I felt like reading. That lasted all of 6th grade. No tests, no questions to answer...just me, a beanbag chair and all the books I could read. Heaven!
It's obviously easier to do this sort of independent work with reading than with math, but a kid can self-check problems. That's got to be better than repeating a grade! Yikes! I'd fight that tooth and nail!
It's utterly anecdotal, and it was 30 years ago or so, but I would say it was an almost wholly positive experience for me!
