I think one of the possible benefits of doing early K is that a kid can get into the habit of being challenged and working. At DSs old preschool he kept hearing the well intended but poisonous comment: "you're so smart". At his new school is Just Another Guy which is a lot healthier for him I think.

This is the whole "Growth Mindset" Carol Dweck thing. I wish I had a link to one of her presentations. I think she makes a lot of good points.