I didn't buy a DS for my daughter. She TOOK MINE. She's been playing it since she was five and still enjoys it... more now that she can read. Oh. .and a perk...
She had a wandering eye. Over the last school year, I was supposed to be patching one eye... I forgot constantly because we were all just so busy and by the time I'd remember it'd be an hour before bedtime.
Took her to the eye doctor a month ago and she commented: You've done an excellent job patching her eye - her eye is far more controlled now! I stared at her and said: But I didn't ... and she said I could've lied to her.

The only thing mine did ... she played the DS for probably an average of an hour a day, fixated on a little figure on the screen bouncing around.