DD went from the middle of K to the middle of 1st and it was fine, although only solved the problem of her being bored with reading/math for about a year. It would have been worse to NOT do it, though.

With DS I would hesitate more because he is too far behind in various areas, like writing, motor skills, the reciprocal conversation thing (he tends to rattle on about topics that don't interest other people), and various other odd quirks.