I agree with CFK. DS6 loved preschool and then loved his half-day, mostly play-based K, but was MISERABLE in his all-day, academic, "learn to read" 1st grade class.
Much depends upon the K program and the child, of course, but our K year went swimmingly. OTOH, we yanked him out of 1st grade for "emergency homeschooling," the educational situation was so glaringly problematic. He'd never been a behavior problem in his life, and he acted up BIG TIME, he was so unhappy and felt so disappointed in school. (And BTW, I had always been one of those people who said, "Oh, I could never homeschool...," so this wasn't a philosophical choice for us.)
Half-day, play-based K classes can be really great for GT kids, since it gives them time to play and be social and learn how to stand in line and all that sort of thing, but they still have the time and energy to pursue learning on their own.
A full-day, highly academic K class is probably something to skip, unless the academics will be tailored to your child's specific needs.
