I had heard all good things about full day K so that's 90% of why we chose it (the other 10% being he was already in preschool 5 hours, and our "half" day option was only 2 hours long... seriously).
K was a disaster in our family from start to finish.
The biggest problem for us was the curriculum. Our son (and all the kids in his preschool) were already reading and doing arithmatic... but in K it was 1 letter per week. Colors and shapes. My son was boooooooored out of his mind.
Our second biggest problem was the length. At 8 hours, plus drive/bus time he was out of the house 9.5 hours a day. He came home *exhausted*. Every bit of self control had already been used up during the day. There was no familiy time (morning madness, school, destress & snack 30min, dinner, bath, bed)... activities were TOO much on top of the ridiculously long day. He was absolutely FALLING APART from the long school day. The first few months were the hardest. Then his spirit just broke. He was too tired. For anything. And what was worse, he was tired and bored. The only part of school he enjoyed (from the kid who used to LOVE learning beyond all other things) was lunch & the bus. He firmly HATED school by christmas. It broke my heart.
I'm certainly not saying this is the case for all kids. But for the 3 that were in his preschool with him who then went onto K with him... all 4 kids were beyond miserable.
One of my biggest regrets with him is that I didn't pull him out over Christmas break. (We now homeschool. Yes, we had other options... but we couldn't afford the 15k per year private school that would have been a perfect fit for him, and putting him in 3rd/4th -the public school's "option" for him and his friends from preschool- would have been social suicide).