My now first grader did fine with it. He had the option for enrichment once a week at his preschool and having a 5 hour day gave me an idea of how he would do. I got him to bed at a reasonable hour and let him play at the playground or chill out at home after school, always with a snack and a drink right away. He made the transition rather well. My second son is starting this year and I'm not sure how he'll do. He still naps (whereas my first son quit napping at two years old) and has a totally different personality. He, too, is doing enrichment at his preschool once a week and seems to be doing okay. I'm also going to try and sign him up for a camp towards the end of summer to get him back into the mindset of being in a group and learning before school starts.
As others have said, there are plenty of extra things to do during full day kindergarten. Recess, gym, art, computers, library, music are all nice breaks from academics.
Marie-Mom to two boys and a girl.