My DS7 skipped K and it has worked out well. In our case, he did two years of 3-day-a-week morning preschool first. We knew he was ready for K early, but didn't think the SD would ever go for it since he has a Thanksgiving birthday, so we just enrolled him in the second year of preschool. However, as we neared K, we realized (partly after speaking with K teachers we knew) that K was no longer right for him. So, after ability and achievement testing and meeting with 1st grade teachers, he skipped K.
Honestly, he had very little difficulty assimilating. He already knew the basics of school from preschool, and was a little familiar with the elementary school because he has older siblings. He did have a bit of difficulty sitting still during circle time and actually sitting at his desk all day (he liked to stand, but not move around), which the teacher attributed to his young age and I attributed to his boredome with the academics.
We gave him several months to adjust to the school day before we brought up the academic mismatch, but the adjustment -- according to us and his teacher -- really only took a few weeks. It was much like it would be for any student new to a school for any reason.
Of course, the academics eventually had to be addressed, and once they were, he was better able to sit still during those better-fit activities.
Anyway, our take on it was that our DS didn't have the same expectations for school that we had, so he just went along with the 1st grade routine without comparing it to what he would have gotten in K. He already knew about standing in lines, sharing, putting on his shoes and coats, etc. from preschool, and those same social skills that he would have gotten in K he was able to use in 1st. Of course, DS is a confident, social kid who never had any problems with separation anxiety or it may have made us a little more wary. I would guess that K teachers might be more accommodating to these kinds of issues than 1st grade teachers, who may be less patient with these insecurities.
Edited to add: DS had good handwriting and liked writing. This is not always the case, particularly with boys. 1st grade did assume he could write decently, so, if he could not, K would likely have been a good choice. Lots to consider!
