We bypassed regular kindergarten. We hit the wall with DS in daycare at 3.5, had to push for early pre-K a few months later, and he blew through that in 4 months even though he was hanging out with the kids 1-2 years older (pre-K and K mix a lot in daycare - when they left at the end of summer, he was left drifting)... the biggest issue we had was boredom and not feeling comfortable with his peers. Teachers loved him and saw no signs of issues (he is so compliant when teachers look at him and he is so charming that peers usually like him, especially the girls, but he was just not happy anymore).
We ended up sending him full time to private GT program at age 4, at DS's insistence for a change, and he was the youngest there but he loved it so much - his classmates were exactly the peers he had been looking for. However, it was a compressed K-1st grade, and he was 2 years younger than he would have been eligible for public K. I can not imagine making him go into public K when he was eligible. But - like HK's DD, this is really a statement of who he is and what he needed, not really an example that you are looking for.
