I bought these new and they are great and they have had them over 2 years now and the internal batteries are still working well.
If you do buy a used one make sure there are no scratches on the screens and if you buy a new one make sure you put the screen protectors on it right away. A lot of the games require touching the screen with the stylus which is pressed too hard could scratch the screen so you want to make sure the screen is protected with the protectors they sell.
there have been some sales on them that includes a game so you may end up getting a new one cheaper when you consider having to buy a game too. Look around as the specials for Christmas as starting and see where the best deal is. Also they will probably be really cheap on the Thanksgiving sale and since the new DSI is out the old ones probably won't be high in demand.
Out of all the pocket size video games my kids have had, the Nintendo DS has been our favorite by far. The batteries last for a long time between charges and they are a lot of fun. I actually enjoy playing it too. They have been wonderful for trips, hospital or doctor visits, or anywhere else they are going to sit for a long time. They are also fun to play multi players and our nieces and nephew will bring theirs over so all 4 of them can play together and don't have to have the same games for each of them. They link together wirelessly and play off 1 game.
My kids hardly go anywhere without them, even if we are going shopping 30 minutes away they will bring them for the car ride.