I sometimes struggle to get my son off the computer as well, but, not so much with the DS.
My mom bought him one for Christmas, and we were very firm when we told him that when it was time to turn it off, it went off. (If he needs to save, he is allowed to get to a save point, but we're watching him play/end play during this time.)
The down side for us has been that the games that we agree on expensive. $30-40 new, and we didn't find anything we wanted 'used' under $20.
For now he only has 2 DS games, I-Spy (which his 4 yr old sister is learning to play) and Super Collapse 3. We're making a list and watching ebay for the games we want.
But, the Gameboy Advance games work in the DS, which he has a few of, and which can be MUCH cheaper on the re-sale. He chose to get Harry Potter and the Chamber of Secrets for $5! It's his current favorite, and it's the one he will play for an hour straight, and looks forward to earning.
Yes, he earns up to an hour of play, seems like a lot, but it gives me some time to be one on one with my daughter. He cooperates with a positive attitude and focuses on school work when he really wants to play. I can't really complain about that. A lot of times we struggle to have a school day like that so if we can be finished with 4 lessons in 2-3 hours (which is average for our really good days), I think he deserves an hour of free time, if he choses to spend it on the DS, I'm cool with that