If you don't already own a DS this is a superb buy. The DS is currently leading the console wars, its outselling all other hand held and home systems and if that isn't enough to get you interested how about this...
The DS features stereo sound, a microphone to interact with games by blowing to help start a fire (Lost in blue) or blow away clouds (Yoshi touch and go), and even speech recognition (Brain training).
The DS features unique touch screen controls which game developers are taking full use of, for example performing operations (Trauma centre) or playing sudoku! The DS also features 2 screens which can be useful for playing on one screen while viewing a map or stats on the other screen.
Many games now even feature a 2 player mode with only 1 cartridge while the 2nd DS will download the multiplayer option.
What else could the DS possibly need you ask? Well it also features internet access to play against player's world wide on games such as Mario Kart and Animal Crossing. There are also download points in various shops where demos can be downloaded to test a game before you actually buy it, or even downloading new packages for your current games! Picross allows owners to download new puzzles to extend the games life.
Finally the future for the DS is also looking pretty good, with connection options to the Wii, the possibilities will be endless!
With all that said, if your willing to buy a DS from new and you have a bit of spare cash, go for the DS Lite, it's a much improved version of this system, however you still can't go wrong with owning a standard DS. However some people also prefer the Bulkiness of this older system as they can be handled easier.