Well, my oldest is in Kindergarten right now and it is all day. His elementary and 1 other elementarty school are the only 2 in our district with all day. This is out of 9 elementaries.I am ALL for all day. I really hope they have it next year so my dd has all day.
Now, like you said...not all kids can handle all day K. I have a friend whose dd is in am K...she is litteraly bouncing off the walls by the time she gets home!!
For my ds, the 1st 2 weeks of school, he was only going in the morning. Then they started the all day program. He had a tough time with it at first,but got used to being their all day. It took about 2 weeks for him to be ok with it. At first, he said he wanted to be in half-day,but he stuck it out and now he loves it!!!

He was 5 when school started,turning 6 in Nov as well!
My personal opinion of all day is that you have to go all day in first grade, why not start them getting used to it in Kindergarten? Does that make sense? What also helped uswas talking to a cousin of mine. She teaches all day K in WI and she told ds what kind of things they do in her class.
I HTH. I think I am all over the place here.