DS4 started half-day K this fall. It was not an easy decision because DS's abilities are extremely asynchronous. His reading and math are at mid- to end of 2nd grade level (we got him tested as was required for early entrance), but his social and physical developments are at (or maybe even below) age level. We made our decision, not based on his intelligence (which we already know will not be adequately stimulated by a kindergarten curriculum), but based on the fact that he enjoys interacting with older children and gets extremely frustrated with kids his age (btw, he does great with much younger children). We just wanted to make sure his first experience with school was fun. So far, he is doing fine. He is having some minor behavioral problems when bored (like throwing his pencil on the floor repeatedly or tearing paper), but not bad. His teacher is really good with him and he loves her.
If you go this route, one thing to keep in mind: you might have to adjust your expectations as you go. You might be surprised that kindergarten is too easy in some ways and also surprised your child is not the best student in some ways. For example, I asked DS today what he is learning and he said, "I don't learn anything in school. Everything is too easy, except for one thing." I got all excited for him and asked him what was hard and he said, "learning to use scissors." I thought that was amusing at first and then I remembered how his handwriting is not very neat. I got a little bit worried about his fine motor development and looked up dysprexia. When I told DH that I was worried that DS has dysprexia, he said, "you have completely lost your perspective. How many 4 year olds do you know who can write, let alone write neatly?" He is right. I have started to think of my son as a kindergartener and without realizing it raised my expectations way up.
