I let my daughter get the DS when she was 6. She had to save her allowance for it and I chipped in to buy a carrying case, car charger (which I recommend, if you have long car rides) and the first game. I was also not so sure about my decision, but I can now say that I think it was a good one. Here's the reasons I think was a good decision:
1. Responsibility. This is a very expensive toy and it took her almost a year to save for it (at $3 a week allowance). She is more careful with it than with other toys.
2. She loves it! Way more than her leapster and v-smile games.
3. It keeps her busy during long car rides.
4. I like to borrow it when she's sleeping :)
The downsides:
1. Expect a marathon DS session when you first get it. Most kids get hooked and don't want to put it away.
2. It does not come with pre-loaded games.
General advice:
1. Read reviews of game on Amazon. Other parents will be able to steer you in the direction of age appropriate games. There's not much fun in watching a 5 year old have a meltdown because the game is more suited for a 10 year old.
2. Again, buy the car charger if you plan to use the game on longer rides.
3. Don't let your child take the game to school. I've heard stories of them being lost, stolen or broken. By me, most kids take their DS to school to have something to do on the bus. I said no to that and don't regret my decision!
Hope this helps-good luck!