I have 6 y/o triplet boys, and they have had ipod touches since they were three. They have helped them learn to read, play games, and do all kinds of age appropriate stuff. More importantly, they can call me with facetime! The first time that happened unexpectedly I was surprised and proud... Now its actually helped in an emergency once when they locked themselves in their room, their mom was outside, and they called me while one of their brothers was climbing out on the roof to get her attention.
We don't allow them in the bedroom after bedtime, before or at school, and occasionally take them away for punishment. I wish we could time-lock them the same way you can OSX, but other than that, they have also been useful in teaching responsibility.