My eldest daughter has had a DS for a couple of years now and i was THE best toy investment we've ever made, never grows bored of it, will replay games, encourages her to save towards new games... it's a good travel toy, it's a good quiet time toy, it can even be a good social toy (as they can link up via wifi mode to other friends with one) on the same note though she doesn't get obsessive over it like i've seen some kids do with video games that require use of the TV, i think the fact she knows she can pick it up and put it down whenever lessens the "must-play-must-play" other consoles have.
She loves HSM game, nintendogs, animal crossings, harvest moon, cooking mama, spyro, chocobo tales- it's had alot of play hours on it.